Wikipedia is a free online encyclopedia created using crowdsourced content. Anyone can make edits to articles. Wikipedia is the largest and most popular general reference work on the Internet and is ranked among the six most popular websites. Wikipedia is owned by the nonprofit Wikimedia Foundation.

Wikipedia was launched on January 15, 2001, by Jimmy Wales and Larry Sanger. Sanger coined its name, a portmanteau of wiki and encyclo pedia. There was only the English language version initially, but it quickly developed similar versions in additional languages, which differ in content and in editing practices. With 5,283,381 articles, English Wikipedia is the largest out of the more than 290 versions of encyclopaedias on Wikipedia. Overall, Wikipedia consists of more than 40 million articles in more than 250 different languages and as of February 2014, it had 18 billion page views and nearly 500 million unique visitors each month.

In 2005, Nature published a peer review comparing 42 science articles from Encyclopædia Britannica and Wikipedia, and found that Wikipedia's level of accuracy approached Encyclopædia Britannica' s. Criticism of Wikipedia includes claims that it exhibits systemic bias, presents a mixture of "truths, half truths, and a few falsehoods", and that in controversial topics, it is subject to manipulation and spin.

Other collaborative online encyclopaedias were attempted before Wikipedia, but none were so successful.

Wikipedia began as a complementary project for Nupedia, a free online English-language encyclopaedia project whose articles were written by experts and reviewed under a formal process. Nupedia was founded on March 9, 2000, under the ownership of Bomis, a web portal company. Its main figures were the Bomis CEO Jimmy Wales and Larry Sanger, editor-in-chief for Nupedia and later Wikipedia. Nupedia was licenced initially under its own Nupedia Open Content License, switching to the GNU Free Documentation License before Wikipedia's founding at the urging of Richard Stallman. Sanger and Wales founded Wikipedia. While Wales is credited with defining the goal of making a publicly editable encyclopedia, Sanger is credited with the strategy of using a wiki to reach that goal. On January 10, 2001, Sanger proposed on the Nupedia mailing list to create a wiki as a "feeder" project for Nupedia.

Wikipedia was launched on January 15, 2001, as a single English-language edition at www.wikipedia.com, and announced by Sanger on the Nupedia mailing list.

Wikipedia's policy of "neutral point-of-view" was codified in its first months.

Otherwise, there were relatively few rules initially and Wikipedia operated independently of Nupedia.

Originally, Bomis intended to make Wikipedia a business for profit.

Wikipedia gained early contributors from Nupedia, Slashdot postings, and web search engine indexing. By August 8, 2001, Wikipedia had over 8,000 articles. On September 25, 2001, Wikipedia had over 13,000 articles. By the end of 2001, it had grown to approximately 20,000 articles and 18 language editions. It had reached 26 language editions by late 2002, 46 by the end of 2003, and 161 by the final days of 2004. Nupedia and Wikipedia coexisted until the former's servers were taken down permanently in 2003, and its text was incorporated into Wikipedia. The English Wikipedia passed the mark of two million articles on September 9, 2007, making it the largest encyclopaedia ever assembled, surpassing even the 1408 Yongle Encyclopedia

Citing fears of commercial advertising and lack of control in Wikipedia, users of the Spanish Wikipedia forked from Wikipedia to create the Enciclopedia Libre in February 2002. These moves encouraged Wales to announce that Wikipedia wouldn't display advertisements, and to change Wikipedia's domain from wikipedia.com to wikipedia.org.

This edition of the encyclopedia, containing 40,000 entries, is now in the public domain; and many of its articles have been used as a basis for articles in Wikipedia.

The Encyclopædia Britannica Eleventh Edition (1910–11) is a 29-volume reference work, an edition of the Encyclopædia Britannica. It was developed during the encyclopaedia's transition from a British to an American publication. Some of its articles were written by the best-known scholars of the time. With the use of automated bots, wikipedia was able to grab all the data and create a backbone for wikipedia.

Several hundred articles from Federal Standard 1037C were imported in February 2002.

The articles were pre-filtered to remove short entries and entries that were from non-public-domain sources.

They were then auto-wikified.

Only articles with titles that did not already exist on Wikipedia were imported.

In August–October 2001, and again in August 2002, contributors imported quite a few entries from the 100 year-old Easton's Bible Dictionary (EBD).

IP 216.99.203.xxx operated by Amillaruploaded the EDB text of 125 articles, starting with Abner.

IP 213.253.39.188, operated by Neil Harris, created 130 articles on 7 August 2002, starting with Ænon.

IP 217.158.106.111created 52 articles on 7 August 2002.

These imports caused some discussion because entries were biased, written in a pedantic Victorian prose, out of date, and some were incorrectly wikified (self links or multiple links).

The issues were worked out with the bot's creator on the Wikipedia mailing list (see [1] and search for Easton's) and various talk pages, and new contributor guidelines were also revised accordingly.

See Wikipedia:Easton's Dictionary topics for the ongoing project, and this list of articles which were not imported.

(October 18, 2002 - October 26, 2002): The so-called rambot operated by Ram-Man created approximately 30,000 U.S. city articles based on U.S. Census tables, at a rate of thousands of articles per day.[2] One error in the data set being used caused a corruption of around 2,000 new articles. Another incident caused the Recent Changes to become more difficult to use for a time and prompted much discussion and the creation of a Wikipedia bot policy [1].

Concurrently with the Rambot, some 800 stub articles on Dutch (500) and Swedish (300) municipalities were created by a bot owned by Jeronimo.

These appear under IP 131.155.230.XXX.

The anomebot was written in 2003 for the purpose of uploading approximately 5000 map images to illustrate the U.S. counties articles.

On Wikipedia in Italian city articles have been uploaded by Gacbot.

This bot has started been used in 2004 and it was used to upload municipalities of Italy, Poland, Botswana, United States, Germany, Finland, Portugal and partially France; the municipalities of France have been edited in a semi-automatic way by User:Gac and uploaded by User:Gac and User:Paginazero [2].

The municipalities of China have been uploaded on Wikidata; data is accessible on several Wikipedias by searching for 郭坑镇 or it can be accessed through Reasonator.

Though the English Wikipedia reached three million articles in August 2009, the growth of the edition, in terms of the numbers of articles and of contributors, appears to have peaked around early 2007.

Around 1,800 articles were added daily to the encyclopaedia in 2006; by 2013 that average was roughly 800.

A team at the Palo Alto Research Center attributed this slowing of growth to the project's increasing exclusivity and resistance to change. Others suggest that the growth is flattening naturally because articles that can be called "low-hanging fruit"—topics that clearly merit an article—have already been created and built up extensively.

In November 2009, a researcher at the Rey Juan Carlos University in Madrid (Spain) found that the English Wikipedia had lost 49,000 editors throughout the first three months of 2009; in comparison, the project lost only 4,900 editors throughout the same period in 2008. The Wall Street Journal cited the array of rules applied to editing and disputes related to such content among the reasons for this trend. Wales disputed these claims in 2009, denying the decline and questioning the methodology of the study. Two years later, Wales acknowledged the presence of a slight decline, noting a decrease from "a little more than 36,000 writers" in June 2010 to 35,800 in June 2011. In the same interview, Wales additionally claimed the number of editors was "stable and sustainable", a claim which was questioned by MIT's Technology Review in a 2013 article titled "The Decline of Wikipedia." In July 2012, the Atlantic reported that the number of administrators is additionally in decline. In the November 25, 2013, issue of New York magazine, Katherine Ward stated "Wikipedia, the sixth-most-used website, is facing an internal crisis. In 2013, MIT's Technology Review revealed that after 2007, the site has lost a third of the volunteer editors who update and correct the online encyclopedia's millions of pages and those still there have focused increasingly on minutiae."

In January 2007, Wikipedia entered for the first time the top-ten list of the most popular websites in the United States, according to comScore Networks. With 42.9 million unique visitors, Wikipedia was ranked number 9, surpassing the New York Times (#10) and Apple (#11). This marked a significant increase over January 2006, when the rank was number 33, with Wikipedia receiving around 18.3 million unique visitors. As of March 2015, Wikipedia has rank 6 among websites in terms of popularity according to Alexa Internet. In 2014, it received 8 billion pageviews every month. On February 9, 2014, The New York Times reported that Wikipedia has 18 billion page views and nearly 500 million unique visitors a month, "according to the ratings firm comScore."

On January 18, 2012, the English Wikipedia participated in a series of coordinated protests against two proposed laws in the United States Congress—the Stop Online Piracy Act (SOPA) and the PROTECT IP Act (PIPA)—by blacking out its pages for 24 hours. More than 162 million people viewed the blackout explanation page that temporarily replaced Wikipedia content.

Loveland and Reagle argue that, in process, Wikipedia follows a long tradition of historical encyclopaedias that accumulated improvements piecemeal through "stigmergic accumulation".

On January 20, 2014, Subodh Varma reporting for The Economic Times indicated that not only had Wikipedia growth flattened but that it has "lost nearly 10 per cent of its page-views last year. That's a decline of about 2 billion between December 2012 and December 2013. Its most popular versions are leading the slide: page-views of the English Wikipedia declined by 12 per cent, those of German version slid by 17 per cent and the Japanese version lost 9 per cent." Varma added that, "While Wikipedia's managers think that this can be due to errors in counting, additional experts feel that Google's Knowledge Graphs project launched last year might be gobbling up Wikipedia users." When contacted on this matter, Clay Shirky, associate professor at New York University and fellow at Harvard's Berkman Center for Internet and Security indicated that he suspected much of the page view decline was due to Knowledge Graphs, stating, "If you can get your question answered from the search page, you don't need to click [any further]."

Unlike traditional encyclopedias, Wikipedia follows the procrastination principle regarding the security of its content. It started almost entirely open—anyone could create articles, and any Wikipedia article can be edited by any reader, even those who didn't have a Wikipedia account. Modifications to all articles would be published immediately. As a result, any article could contain inaccuracies such as errors, ideological biases, and nonsensical or irrelevant text.

Over time, the English Wikipedia and a few additional Wikipedias gradually have restricted modifications.

For example, in the English Wikipedia and a few additional language editions, only registered users might create a new article.

On the English Wikipedia and a few others, a few particularly controversial, sensitive and/or vandalism-prone pages are now "protected" to a few degree.

A frequently vandalised article can be semi-protected , meaning that only certain editors are able to modify it. A particularly contentious article might be locked so that only administrators are able to make changes.

In certain cases, all editors are allowed to submit modifications, but review is required for a few editors, depending on certain conditions.

For example, the German Wikipedia maintains "stable versions" of articles, which have passed certain reviews. Following protracted trials and community discussion, the English Wikipedia introduced the "pending changes" system in December 2012. Under this system, new users' edits to certain controversial or vandalism-prone articles are "subject to review from an established Wikipedia editor before publication".

Although changes aren't systematically reviewed, the software that powers Wikipedia provides certain tools allowing anyone to review changes made by others.

The "History" page of each article links to each revision.

On most articles, anyone can undo others' changes by clicking a link on the article's history page.

Anyone can view the latest changes to articles, and anyone might maintain a "watchlist" of articles that interest them so they can be notified of any changes. "New pages patrol" is a process whereby newly created articles are checked for obvious problems.

In 2003, economics PhD student Andrea Ciffolilli argued that the low transaction costs of participating in a wiki create a catalyst for collaborative development, and that features such as allowing easy access to past versions of a page favour "creative construction" over "creative destruction".

Any edit that changes content in a way that deliberately compromises the integrity of Wikipedia is considered vandalism.

The most common and obvious types of vandalism include insertion of obscenities and crude humor.

Vandalism can additionally include advertising language and additional types of spam.

Sometimes editors commit vandalism by removing information or entirely blanking a given page.

Less common types of vandalism, such as the deliberate addition of plausible but false information to an article, can be more difficult to detect.

Vandals can introduce irrelevant formatting, modify page semantics such as the page's title or categorization, manipulate the underlying code of an article, or use images disruptively.

Obvious vandalism is generally easy to remove from wiki articles; the median time to detect and fix vandalism is a few minutes.

Notwithstanding a few vandalism takes much longer to repair.

In the Seigenthaler biography incident, an anonymous editor introduced false information into the biography of American political figure John Seigenthaler in May 2005. Seigenthaler was falsely presented as a suspect in the assassination of John F. Kennedy. The article remained uncorrected for four months. Seigenthaler, the founding editorial director of USA Today and founder of the Freedom Forum First Amendment Center at Vanderbilt University, called Wikipedia co-founder Jimmy Wales and asked whether he'd any way of knowing who contributed the misinformation. Wales replied that he did not, although the perpetrator was eventually traced. After the incident, Seigenthaler described Wikipedia as "a flawed and irresponsible research tool". This incident led to policy changes at Wikipedia, specifically targeted at tightening up the verifiability of biographical articles of living people.

Content in Wikipedia is subject to the laws (in particular, copyright laws) of the United States and of the U.S. state of Virginia, where the majority of Wikipedia's servers are located. Beyond legal matters, the editorial principles of Wikipedia are embodied in the "five pillars" and in numerous policies and guidelines intended to appropriately shape content. Even these rules are stored in wiki form, and Wikipedia editors write and revise the website's policies and guidelines. Editors can enforce these rules by deleting or modifying non-compliant material. Originally, rules on the non-English editions of Wikipedia were based on a translation of the rules for the English Wikipedia. They have after diverged to a few extent.

According to the rules on the English Wikipedia, each entry in Wikipedia must be about a topic that's encyclopedic and isn't a dictionary entry or dictionary-like. A topic should additionally meet Wikipedia's standards of "notability", which generally means that the topic must have been covered in mainstream media or major academic journal sources that are independent of the article's subject. Further, Wikipedia intends to convey only knowledge that's already established and recognized. It mustn't present original research. A claim that's likely to be challenged requires a reference to a reliable source. Among Wikipedia editors, this is often phrased as "verifiability, not truth" to express the idea that the readers, not the encyclopedia, are ultimately responsible for checking the truthfulness of the articles and making their own interpretations. This can at times lead to the removal of information that's valid. Finally, Wikipedia mustn't take sides. All opinions and viewpoints, if attributable to external sources, must enjoy an appropriate share of coverage within an article. This is known as neutral point of view (NPOV).

Wikipedia's initial anarchy integrated democratic and hierarchical elements over time. An article isn't considered to be owned by its creator or any additional editor and isn't vetted by any recognised authority. Wikipedia's contributors avoid a tragedy of the commons by internalising benefits. They do this by experiencing flow and identifying with and gaining status in the Wikipedia community.

Editors in good standing in the community can run for one of a large number of levels of volunteer stewardship: this begins with "administrator", privileged users who can delete pages, prevent articles from being changed in case of vandalism or editorial disputes, and try to prevent certain persons from editing. Despite the name, administrators aren't supposed to enjoy any special privilege in decision-making; instead, their powers are mostly limited to making edits that have project-wide effects and thus are disallowed to ordinary editors, and to implement restrictions intended to prevent certain persons from making disruptive edits (such as vandalism).

Fewer editors become administrators than in years past, in part because the process of vetting potential Wikipedia administrators has become more rigorous.

Bureaucrats name new administrators, solely upon the recommendations from the community.

Wikipedians might dispute, for example by repeatedly making opposite changes to an article.

Over time, Wikipedia has developed documentation for editors about dispute resolution.

In order to determine community consensus, editors can raise issues at the Village Pump, or initiate a request for comment.

The Arbitration Committee presides over the ultimate dispute resolution process.

Although disputes usually arise from a disagreement between two opposing views on how an article should read, the Arbitration Committee explicitly refuses to directly rule on the specific view that should be adopted.

Statistical analyses suggest that the committee ignores the content of disputes and rather focuses on the way disputes are conducted, functioning not so much to resolve disputes and make peace between conflicting editors, but to weed out problematic editors while allowing potentially productive editors back in to participate.

Therefore, the committee doesn't dictate the content of articles, although it at times condemns content changes when it deems the new content violates Wikipedia policies (for example, if the new content is considered biased). Its remedies include cautions and probations (used in 63 percent of cases) and banning editors from articles (43%), subject matters (23%) or Wikipedia (16%). Complete bans from Wikipedia are generally limited to instances of impersonation and anti-social behavior. When conduct isn't impersonation or anti-social, but rather anti-consensus or in violation of editing policies, remedies tend to be limited to warnings.

Each article and each user of Wikipedia has an associated "Talk" page.

These form the primary communication channel for editors to discuss, coordinate and debate.

Wikipedia's community has been described as cult-like, although not always with entirely negative connotations. The project's preference for cohesiveness, even if it requires compromise that includes disregard of credentials, has been referred to as "anti-elitism".

Wikipedians at times award one another virtual barnstars for good work. These personalised tokens of appreciation reveal a wide range of valued work extending far beyond simple editing to include social support, administrative actions, and types of articulation work.

Wikipedia doesn't require that its editors and contributors provide identification.

As Wikipedia grew, "Who writes Wikipedia?" became one of the questions frequently asked on the project.

Jimmy Wales once argued that only "a community... a dedicated group of a few hundred volunteers" makes the bulk of contributions to Wikipedia and that the project is therefore "much like any traditional organization".

In 2008, a Slate magazine article reported that: "According to researchers in Palo Alto, 1 percent of Wikipedia users are responsible for about half of the site's edits." This method of evaluating contributions was later disputed by Aaron Swartz, who noted that several articles he sampled had large portions of their content (measured by number of characters) contributed by users with low edit counts.

The English Wikipedia has 5,283,381 articles, 29,510,643 registered editors, and 125,400 active editors. An editor is considered active if they have made one or more edits in the past thirty days.

Editors who fail to comply with Wikipedia cultural rituals, such as signing talk page comments, might implicitly signal that they're Wikipedia outsiders, increasing the odds that Wikipedia insiders might target or discount their contributions.

Becoming a Wikipedia insider involves non-trivial costs: the contributor is expected to learn Wikipedia-specific technological codes, submit to a at times convoluted dispute resolution process, and learn a "baffling culture rich with in-jokes and insider references".

Editors who don't log in are in a few sense second-class citizens on Wikipedia, as "participants are accredited by members of the wiki community, who have a vested interest in preserving the quality of the work product, on the basis of their ongoing participation", but the contribution histories of anonymous unregistered editors recognised only by their IP addresses can't be attributed to a particular editor with certainty.

A 2007 study by researchers from Dartmouth College found that "anonymous and infrequent contributors to Wikipedia […] are as reliable a source of knowledge as those contributors who register with the site". Jimmy Wales stated in 2009 that "(I)t turns out over fifty percent of all the edits are done by just.7% of the users... 524 people... And in fact the most active 2%, which is 1400 people, have done 73.4% of all the edits." Notwithstanding Business Insider editor and journalist Henry Blodget showed in 2009 that in a random sample of articles, most content in Wikipedia (measured by the amount of contributed text that survives to the latest sampled edit) is created by "outsiders", while most editing and formatting is done by "insiders".

A 2008 study found that Wikipedians were less agreeable, open, and conscientious than others, although a later commentary pointed out serious flaws, including that the data showed higher openness, that the differences with the control group were small as were the samples.

According to a 2009 study, there's "evidence of growing resistance from the Wikipedia community to new content".

One study found that the contributor base to Wikipedia "was barely thirteen percent women; the average age of a contributor was in the mid-20s".

A 2011 study by researchers from the University of Minnesota found that females comprised 16.1% of the 38,497 editors who started editing Wikipedia throughout 2009. In a January 2011 New York Times article, Noam Cohen observed that just thirteen percent of Wikipedia's contributors are female according to a 2008 Wikimedia Foundation survey. Sue Gardner, a former executive director of the Wikimedia Foundation, hoped to see female contributions increase to twenty-five percent by 2015. Linda Basch, president of the National Council for Research on Women, noted the contrast in these Wikipedia editor statistics with the percentage of women currently completing bachelor's degrees, master's degrees and PhD programmes in the United States (all at rates of 50 percent or greater).

In response, various universities have hosted edit-a-thons to encourage more women to participate in the Wikipedia community. In fall 2013, 15 colleges and universities, including Yale, Brown, and Pennsylvania State, offered college credit for students to "write feminist thinking" about technology into Wikipedia.

In August 2014, Wikipedia co-founder Jimmy Wales said in a BBC interview the Wikimedia Foundation's was "...really doubling down our efforts..." to reach twenty-five percent of female editors (originally targeted by 2015), after the Foundation had "totally failed" so far. Wales said "a lot of things need to happen.. a lot of outreach, a lot of software changes".

There are currently 295 language editions of Wikipedia (also called language versions, or simply Wikipedias). Thirteen of these have over one million articles each (English, Swedish, Cebuano, German, Dutch, French, Russian, Italian, Spanish, Waray-Waray, Polish, Vietnamese and Japanese), five more have over 500,000 articles (Portuguese, Chinese, Ukrainian, Catalan and Persian), 40 more have over 100,000 articles, and 76 more have over 10,000 articles. The largest, the English Wikipedia, has over 5.2 million articles. As of October 2016, according to Alexa, the English subdomain (en.wikipedia.org; English Wikipedia) receives approximately 57 percent of Wikipedia's cumulative traffic, with the remaining split among the additional languages (Russian: 8%; Spanish: 7%; Japanese: 7%; German: 4%). As of November 2016, the six largest language editions are (in order of article count) the English, Swedish, Cebuano, German, Dutch, and French Wikipedias.

The unit for the numbers in bars is articles. Since Wikipedia is based on the Web and therefore worldwide, contributors to the same language edition might use different dialects or might come from different countries (as is the case for the English edition). These differences might lead to a few conflicts over spelling differences (e.g. colour versus color

Though the various language editions are held to global policies such as "neutral point of view", they diverge on a few points of policy and practice, most notably on whether images that aren't licensed freely might be used under a claim of fair use.

Jimmy Wales has described Wikipedia as "an effort to create and distribute a free encyclopaedia of the highest possible quality to every single person on the planet in their own language".

Though each language edition functions more or less independently, a few efforts are made to supervise them all.

They are coordinated in part by Meta-Wiki, the Wikimedia Foundation's wiki devoted to maintaining all of its projects (Wikipedia and others).

For instance, Meta-Wiki provides important statistics on all language editions of Wikipedia, and it maintains a list of articles every Wikipedia should have.

The list concerns basic content by subject: biography, history, geography, society, culture, science, technology, and mathematics.

As for the rest, it isn't rare for articles strongly related to a particular language not to have counterparts in another edition.

For example, articles about small towns in the United States might only be available in English, even when they meet notability criteria of additional language Wikipedia projects.

Translated articles represent only a small portion of articles in most editions, in part because fully automated translation of articles is disallowed.

Articles available in more than one language might offer "interwiki links", which link to the counterpart articles in additional editions.

A study published by PLOS ONE in 2012 additionally estimated the share of contributions to different editions of Wikipedia from different regions of the world. It reported that the proportion of the edits made from North America was 51 percent for the English Wikipedia, and twenty-five percent for the simple English Wikipedia. The Wikimedia Foundation hopes to increase the number of editors in the Global South to 37 percent by 2015.

On March 1, 2014, The Economist in an article titled "The Future of Wikipedia" cited a trend analysis concerning data published by Wikimedia stating that: "The number of editors for the English-language version has fallen by a third in seven years." The attrition rate for active editors in English Wikipedia was cited by The Economist as substantially in contrast to statistics for Wikipedia in additional languages (non-English Wikipedia). The Economist reported that the number of contributors with an average of five of more edits per month was relatively constant after 2008 for Wikipedia in additional languages at approximately 42,000 editors within narrow seasonal variances of about 2,000 editors up or down. The attrition rates for editors in English Wikipedia, by sharp comparison, were cited as peaking in 2007 at approximately 50,000 editors which has dropped to 30,000 editors as of the start of 2014. At the quoted trend rate, the number of active editors in English Wikipedia has lost approximately 20,000 editors to attrition after 2007, and the documented trend rate indicates the loss of another 20,000 editors by 2021, down to 10,000 active editors on English Wikipedia by 2021 if left unabated. Given that the trend analysis published in The Economist presents the number of active editors for Wikipedia in additional languages (non-English Wikipedia) as remaining relatively constant and successful in sustaining its numbers at approximately 42,000 active editors, the contrast has pointed to the effectiveness of Wikipedia in additional languages to retain its active editors on a renewable and sustained basis. No comment was made concerning which of the differentiated edit policy standards from Wikipedia in additional languages (non-English Wikipedia) would provide a possible alternative to English Wikipedia for effectively ameliorating substantial editor attrition rates on the English language Wikipedia.

Several Wikipedians have criticized Wikipedia's large and growing regulation, which includes over 50 policies and nearly 150,000 words as of 2014.

Critics have stated that Wikipedia exhibits systemic bias. Columnist and journalist Edwin Black criticises Wikipedia for being a mixture of "truth, half truth, and a few falsehoods". Articles in The Chronicle of Higher Education and The Journal of Academic Librarianship have criticised Wikipedia's Undue Weight policy, concluding that the fact that Wikipedia explicitly isn't designed to provide correct information about a subject, but rather focus on all the major viewpoints on the subject and give less attention to minor ones, which creates omissions that can lead to false beliefs based on incomplete information.

Journalists Oliver Kamm and Edwin Black noted how articles are dominated by the loudest and most persistent voices, usually by a group with an "ax to grind" on the topic. An article in Education Next Journal concluded that as a resource about controversial topics, Wikipedia is notoriously subject to manipulation and spin.

In 2006, the Wikipedia Watch criticism website listed dozens of examples of plagiarism in the English Wikipedia.

Articles for traditional encyclopaedias such as Encyclopædia Britannica are carefully and deliberately written by experts, lending such encyclopaedias a reputation for accuracy. Conversely, Wikipedia is often cited for factual inaccuracies and misrepresentations. Notwithstanding a peer review in 2005 of 42 scientific entries on both Wikipedia and Encyclopædia Britannica by the science journal Nature found few differences in accuracy, and concluded that "the average science entry in Wikipedia contained around four inaccuracies; Britannica, about three." Reagle suggested that while the study reflects "a topical strength of Wikipedia contributors" in science articles, "Wikipedia might not have fared so well using a random sampling of articles or on humanities subjects." The findings by Nature were disputed by Encyclopædia Britannica, and in response, Nature gave a rebuttal of the points raised by Britannica. In addition to the point-for-point disagreement between these two parties, others have examined the sample size and selection method used in the Nature effort, and suggested a "flawed study design" (in Nature' s manual selection of articles, in part or in whole, for comparison), absence of statistical analysis (e.g., of reported confidence intervals), and a lack of study "statistical power" (i.e., owing to small sample size, 42 or 4 x 10 1 articles compared, vs >10 5 and >10 6 set sizes for Britannica and the English Wikipedia, respectively).

As a consequence of the open structure, Wikipedia "makes no guarantee of validity" of its content, after no one is ultimately responsible for any claims appearing in it.

Concerns have been raised by PC World in 2009 regarding the lack of accountability that results from users' anonymity, the insertion of false information, vandalism, and similar problems.

Economist Tyler Cowen wrote: "If I'd to guess whether Wikipedia or the median refereed journal article on economics was more likely to be true, after a not so long think I would opt for Wikipedia." He comments that a few traditional sources of non-fiction suffer from systemic biases and novel results, in his opinion, are over-reported in journal articles and relevant information is omitted from news reports. Notwithstanding he additionally cautions that errors are frequently found on Internet sites, and that academics and experts must be vigilant in correcting them.

Critics argue that Wikipedia's open nature and a lack of proper sources for most of the information makes it unreliable.

Some commentators suggest that Wikipedia might be reliable, but that the reliability of any given article isn't clear.

Editors of traditional reference works such as the Encyclopædia Britannica have questioned the project's utility and status as an encyclopedia.

Wikipedia's open structure inherently makes it an easy target for Internet trolls, spammers, and various forms of paid advocacy seen as counterproductive to the maintenance of a neutral and verifiable online encyclopedia. In response to paid advocacy editing and undisclosed editing issues, Wikipedia was reported in an article by Jeff Elder in The Wall Street Journal on June 16, 2014, to have strengthened its rules and laws against undisclosed editing. The article stated that: "Beginning Monday [from date of article], changes in Wikipedia's terms of use will require anyone paid to edit articles to disclose that arrangement. Katherine Maher, the nonprofit Wikimedia Foundation's chief communications officer, said the changes address a sentiment among volunteer editors that, 'we're not an advertising service; we're an encyclopedia.'" These issues, among others, had been parodied after the first decade of Wikipedia, notably by Stephen Colbert on The Colbert Report

Most university lecturers discourage students from citing any encyclopaedia in academic work, preferring primary sources; a few specifically prohibit Wikipedia citations. Wales stresses that encyclopaedias of any type aren't usually appropriate to use as citeable sources, and shouldn't be relied upon as authoritative. Wales once (2006 or earlier) said he receives about ten emails weekly from students saying they got failing grades on papers because they cited Wikipedia; he told the students they got what they deserved. "For God's sake, you're in college; don't cite the encyclopedia", he said.

In February 2007, an article in The Harvard Crimson newspaper reported that a few of the professors at Harvard University were including Wikipedia articles in their syllabi, although without realising the articles might change. In June 2007, former president of the American Library Association Michael Gorman condemned Wikipedia, along with Google, stating that academics who endorse the use of Wikipedia are "the intellectual equivalent of a dietitian who recommends a steady diet of Big Macs with everything".

A Harvard law textbook, Legal Research in a Nutshell (2011), cites Wikipedia as a "general source" that "can be a real boon" in "coming up to speed in the law governing a situation" and, "while not authoritative, can provide basic facts as well as leads to more in-depth resources".

On March 5, 2014, Julie Beck writing for The Atlantic magazine in an article titled "Doctors' #1 Source for Healthcare Information: Wikipedia", stated that "Fifty percent of physicians look up conditions on the (Wikipedia) site, and a few are editing articles themselves to improve the quality of available information." Beck continued to detail in this article new programmes of Dr. Amin Azzam at the University of San Francisco to offer medical school courses to medical students for learning to edit and improve Wikipedia articles on health-related issues, as well as internal quality control programmes within Wikipedia organised by Dr. James Heilman to improve a group of 200 health-related articles of central medical importance up to Wikipedia's highest standard of articles using its Featured Article and Good Article peer review evaluation process. In a May 7, 2014, follow-up article in The Atlantic titled "Can Wikipedia Ever Be a Definitive Medical Text?", Julie Beck quotes Wikiproject Medicine's Dr. James Heilman as stating: "Just because a reference is peer-reviewed doesn't mean it's a high-quality reference." Beck added that: "Wikipedia has its own peer review process before articles can be classified as 'good' or 'featured.' Heilman, who has participated in that process before, says 'less than 1 percent' of Wikipedia's medical articles have passed.

In 2008, researchers at Carnegie Mellon University found that the quality of a Wikipedia article would suffer rather than gain from adding more writers when the article lacked appropriate explicit or implicit coordination. For instance, when contributors rewrite small portions of an entry rather than making full-length revisions, high- and low-quality content might be intermingled within an entry. Roy Rosenzweig, a history professor, stated that American National Biography Online outperformed Wikipedia in terms of its "clear and engaging prose", which, he said, was an important aspect of good historical writing. Contrasting Wikipedia's treatment of Abraham Lincoln to that of Civil War historian James McPherson in American National Biography Online, he said that both were essentially accurate and covered the major episodes in Lincoln's life, but praised "McPherson's richer contextualization […] his artful use of quotations to capture Lincoln's voice […] and […] his ability to convey a profound message in a handful of words." By contrast, he gives an example of Wikipedia's prose that he finds "both verbose and dull". Rosenzweig additionally criticised the "waffling—encouraged by the npov policy—[which] means that it is hard to discern any overall interpretive stance in Wikipedia history". By example, he quoted the conclusion of Wikipedia's article on William Clarke Quantrill. While generally praising the article, he pointed out its "waffling" conclusion: "Some historians […] remember him as an opportunistic, bloodthirsty outlaw, while others continue to view him as a daring soldier and local folk hero."

Other critics have made similar charges that, even if Wikipedia articles are factually accurate, they're often written in a poor, almost unreadable style.

Frequent Wikipedia critic Andrew Orlowski commented: "Even when a Wikipedia entry is 100 per cent factually correct, and those facts have been carefully chosen, it all too often reads as if it has been translated from one language to another then into to a third, passing an illiterate translator at each stage."

A study of articles on cancer was undertaken in 2010 by Yaacov Lawrence of the Kimmel Cancer Center at Thomas Jefferson University limited to those Wikipedia articles which can be found in the Physician Data Query and excluding Wikipedia articles written at the "start" class or the "stub" class level. Lawrence found the articles accurate but not quite readable, and thought that "Wikipedia's lack of readability (to non-college readers) might reflect its varied origins and haphazard editing". The Economist argued that better-written articles tend to be more reliable: "inelegant or ranting prose usually reflects muddled thoughts and incomplete information".

Wikipedia seeks to create a summary of all human knowledge in the form of an online encyclopedia, with each topic covered encyclopedically in one article. Since it has terabytes of disc space, it can have far more topics than can be covered by any printed encyclopedia. The exact degree and manner of coverage on Wikipedia is under constant review by its editors, and disagreements aren't uncommon (see deletionism and inclusionism). Wikipedia contains materials that a few people might find objectionable, offensive, or pornographic because Wikipedia isn't censored. The policy has at times proved controversial: in 2008, Wikipedia rejected an online petition against the inclusion of images of Muhammad in the English edition of its Muhammad article, citing this policy. The presence of politically, religiously, and pornographically sensitive materials in Wikipedia has led to the censorship of Wikipedia by national authorities in China, Pakistan, and the United Kingdom, among additional countries.

A 2008 study conducted by researchers at Carnegie Mellon University and Palo Alto Research Center gave a distribution of topics as well as growth (from July 2006 to January 2008) in each field:

These numbers refer only to the quantity of articles: it is possible for one topic to contain a large number of short articles and another to contain a small number of large ones.

Through its "Wikipedia Loves Libraries" program, Wikipedia has partnered with major public libraries such as the New York Public Library for the Performing Arts to expand its coverage of underrepresented subjects and articles.

A 2011 study conducted by researchers at the University of Minnesota indicated that male and female editors focus on different coverage topics. There was a greater concentration of females in the People and Arts category, while males focus more on Geography and Science.

Research conducted by Mark Graham of the Oxford Internet Institute in 2009 indicated that the geographic distribution of article topics is highly uneven. Africa is most underrepresented.

An editorial in The Guardian in 2014 noted that women porn stars are better covered than women writers as a further example.

When multiple editors contribute to one topic or set of topics, systemic bias might arise, due to the demographic backgrounds of the editors. In 2011, Wales noted that the unevenness of coverage is a reflection of the demography of the editors, which predominantly consists of young males with high education levels in the developed world (cf. previously). The October 22, 2013 essay by Tom Simonite in MIT's Technology Review titled "The Decline of Wikipedia" discussed the effect of systemic bias and policy creep on the.

Systemic bias on Wikipedia might follow that of culture generally, for example favouring certain nationalities, ethnicities or majority religions. It might more specifically follow the biases of Internet culture, inclining to being young, male, English-speaking, educated, technologically aware, and wealthy enough to spare time for editing. Biases of its own might include over-emphasis on topics such as pop culture, technology, and current events.

Feminist writer TaraElla, founder of the WikiEqualize project, believes that the Notability Criteria must be abolished for the sake of equality, and has suggested that a Cultural Contribution Criteria be used instead. Such a criteria would not be dependent on popularity or attention in mainstream media, but rather on contribution to human culture as a whole. She has written extensively on what such a criteria would look like, and how it can work.

Taha Yasseri of the University of Oxford, in 2013, studied the statistical trends of systemic bias at Wikipedia introduced by editing conflicts and their resolution. His research examined the counterproductive work behavior of edit warring. Yasseri contended that simple reverts or "undo" operations weren't the most significant measure of counterproductive behaviour at Wikipedia and relied instead on the statistical measurement of detecting "reverting/reverted pairs" or "mutually reverting edit pairs." Such a "mutually reverting edit pair" is defined where one editor reverts the edit of another editor who then, in sequence, returns to revert the first editor in the "mutually reverting edit pairs." The results were tabulated for several language versions of Wikipedia. The English Wikipedia's three largest conflict rates belonged to the articles George W. Bush , Anarchism and Muhammad . By comparison, for the German Wikipedia, the three largest conflict rates at the time of the Oxford study were for the articles covering (i) Croatia , (ii) Scientology and (iii) 9/11 conspiracy theories

Wikipedia has been criticised for allowing information of graphic content.

Articles depicting arguably objectionable content (such as Feces , Cadaver , Human penis , Vulva , and Nudity

The site additionally includes sexual content such as images and videos of masturbation and ejaculation, photographs of nude children, illustrations of zoophilia, and photos from hardcore pornographic films in its articles.

The Wikipedia article about Virgin Killer — a 1976 album from German heavy metal band Scorpions —features a picture of the album's original cover, which depicts a naked prepubescent girl. The original release cover caused controversy and was replaced in a few countries. In December 2008, access to the Wikipedia article Virgin Killer was blocked for four days by most Internet service providers in the United Kingdom after it was reported by a member of the public as child pornography, to the Internet Watch Foundation (IWF), which issues a stop list to Internet service providers. IWF, a non-profit, non-government-affiliated organization, later criticised the inclusion of the picture as "distasteful".

In April 2010, Sanger wrote a letter to the Federal Bureau of Investigation, outlining his concerns that two categories of images on Wikimedia Commons contained child pornography, and were in violation of US federal obscenity law. Sanger later clarified that the images, which were related to pedophilia and one about lolicon, weren't of real children, but said that they constituted "obscene visual representations of the sexual abuse of children", under the PROTECT Act of 2003. That law bans photographic child pornography and cartoon images and drawings of children that are obscene under American law. Sanger additionally expressed concerns about access to the images on Wikipedia in schools. Wikimedia Foundation spokesman Jay Walsh strongly rejected Sanger's accusation, saying that Wikipedia didn't have "material we would deem to be illegal. If we did, we would remove it." Following the complaint by Sanger, Wales deleted sexual images without consulting the community. After a few editors who volunteer to maintain the site argued that the decision to delete had been made hastily, Wales voluntarily gave up a few of the powers he'd held up to that time as part of his co-founder status. He wrote in a message to the Wikimedia Foundation mailing-list that this action was "in the interest of encouraging this discussion to be about real philosophical/content issues, rather than be about me and how quickly I acted". Critics, including Wikipediocracy, noticed that a large number of of the pornographic images deleted from Wikipedia after 2010 have reappeared.

One privacy concern in the case of Wikipedia is the right of a private citizen to remain a "private citizen" rather than a "public figure" in the eyes of the law. It is a battle between the right to be anonymous in cyberspace and the right to be anonymous in real life ("meatspace"). A particular problem occurs in the case of an individual who's relatively unimportant and for whom there exists a Wikipedia page against her or his wishes.

In January 2006, a German court ordered the German Wikipedia shut down within Germany because it stated the full name of Boris Floricic, aka "Tron", a deceased hacker. On February 9, 2006, the injunction against Wikimedia Deutschland was overturned, with the court rejecting the notion that Tron's right to privacy or that of his parents was being violated.

Wikipedia has a " Volunteer Response Team " that uses the OTRS system to handle queries without having to reveal the identities of the involved parties. This is used, for example, in confirming the permission for using individual images and additional media in the project.

Wikipedia has been described as harbouring a battleground culture of sexism and harassment. The perceived toxic attitudes and tolerance of violent and abusive language are additionally reasons put forth for the gender gap in Wikipedia editors.

A group of Wikipedia editors might form a WikiProject to focus their work on a specific topic area, using its associated discussion page to coordinate changes across multiple articles.

Wikipedia is hosted and funded by the Wikimedia Foundation, a non-profit organisation which additionally operates Wikipedia-related projects such as Wiktionary and Wikibooks. The foundation relies on public contributions and grants to fund its mission. The foundation's 2013 IRS Form 990 shows revenue of $39.7 million and expenses of almost $29 million, with assets of $37.2 million and liabilities of about $2.3 million.

In May 2014, Wikimedia Foundation named Lila Tretikov as its new executive director, taking over for Sue Gardner. The Wall Street Journal reported on May 1, 2014, that Tretikov's information technology background from her years at University of California offers Wikipedia an opportunity to develop in more concentrated directions guided by her often repeated position statement that, "Information, like air, wants to be free." The same Wall Street Journal article reported these directions of development according to an interview with spokesman Jay Walsh of Wikimedia who "said Tretikov would address that issue (paid advocacy) as a priority. 'We are really pushing toward more transparency... We are reinforcing that paid advocacy isn't welcome.' Initiatives to involve greater diversity of contributors, better mobile support of Wikipedia, new geo-location tools to find local content more easily, and more tools for users in the second and third world are additionally priorities, Walsh said."

Wikipedia is additionally supported by a large number of organisations and groups that are affiliated with the Wikimedia Foundation but independently-run, called Wikimedia movement affiliates. These include Wikimedia chapters (which are national or sub-national organizations, such as Wikimedia Deutschland and Wikimédia France), thematic organisations (such as Amical Wikimedia for the Catalan language community), and user groups. These affiliates participate in the promotion, development, and funding of Wikipedia.

The operation of Wikipedia depends on MediaWiki, a custom-made, free and open source wiki software platform written in PHP and built upon the MySQL database system. The software incorporates programming features such as a macro language, variables, a transclusion system for templates, and URL redirection. MediaWiki is licenced under the GNU General Public License and it is used by all Wikimedia projects, as well as a large number of additional wiki projects. Originally, Wikipedia ran on UseModWiki written in Perl by Clifford Adams (Phase I), which initially required CamelCase for article hyperlinks; the present double bracket style was incorporated later. Starting in January 2002 (Phase II), Wikipedia began running on a PHP wiki engine with a MySQL database; this software was custom-made for Wikipedia by Magnus Manske. The Phase II software was repeatedly modified to accommodate the exponentially increasing demand. In July 2002 (Phase III), Wikipedia shifted to the third-generation software, MediaWiki, originally written by Lee Daniel Crocker.

Several MediaWiki extensions are installed to extend the functionality of the MediaWiki software.

In April 2005, a Lucene extension was added to MediaWiki's built-in search and Wikipedia switched from MySQL to Lucene for searching. The site currently uses Lucene Search 2.1, which is written in Java and based on Lucene library 2.3.

In July 2013, after extensive beta testing, a WYSIWYG (What You See Is What You Get) extension, VisualEditor, was opened to public use. It was met with much rejection and criticism, and was described as "slow and buggy". The feature was turned off afterward.

Computer programmes called bots have been used widely to perform simple and repetitive tasks, such as correcting common misspellings and stylistic issues, or to start articles such as geography entries in a standard format from statistical data. One controversial contributor massively creating articles with his bot was reported to create up to ten thousand articles on the Swedish Wikipedia on certain days. There are additionally a few bots designed to automatically warn editors making common editing errors (such as unmatched quotes or unmatched parenthesis). Edits misidentified by a bot as the work of a banned editor can be restored by additional editors. An anti-vandal bot tries to detect and revert vandalism quickly and automatically. Bots can additionally report edits from particular accounts or IP address ranges, as was done at the time of the MH17 jet downing incident in July 2014. Bots on Wikipedia must be approved prior to activation.

According to Andrew Lih, the current expansion of Wikipedia to millions of articles would be difficult to envision without the use of such bots.

A "WikiProject" is a group of contributors who want to work together as a team to improve Wikipedia. These groups often focus on a specific topic area (for example, women's history), a specific location or a specific kind of task (for example, checking newly created pages). The English Wikipedia currently has over 2,000 WikiProjects and activity varies.

In 2007, in preparation for producing a print version, the English Wikipedia introduced an assessment scale of the quality of articles.

Articles are rated by Wikiprojects.

The range of quality classes begins with "Stub" (very short pages), followed by "Start", "C" and "B" (in increasing order of quality).

Community peer review is needed for the article to enter one of the highest quality classes: either "A", "good article" or the highest, "featured article". Of the about 4.4 million articles and lists assessed as of March 2015, a little more than 5000 (0.12%) are featured articles, and a little less than 2000 (0.04%) are featured lists. One featured article per day, as selected by editors, appears on the main page of Wikipedia.

The articles can additionally be rated as per "importance" as judged by a Wikiproject.

Currently, there are 5 importance categories: "low", "mid", "high", "top", and "???" for unclassified/unsure level.

For a particular article, different Wikiprojects might assign different importance levels.

The Wikipedia Version 1.0 Editorial Team has developed a table (shown below) that displays data of all rated articles by quality and importance, on the English Wikipedia. If an article or list receives different ratings by two or more Wikiprojects, then the highest rating is used in the table, pie-charts, and bar-chart. The software regularly auto-updates the data.

Researcher Giacomo Poderi found that articles tend to reach featured status via the intensive work of a few editors.

A 2010 study found unevenness in quality among featured articles and concluded that the community process is ineffective in assessing the quality of articles.

[Note: The table above (prepared by the Wikipedia Version 1.0 Editorial Team) is automatically updated daily by User:WP 1.0 bot, but the bar-chart and the two pie-charts aren't auto-updated. In them, new data has to be entered by a Wikipedia editor.]

Wikipedia receives between 25,000 and 60,000 page requests per second, depending on time of day.

As of 2008 page requests are first passed to a front-end layer of Squid caching servers. Further statistics, based on a publicly available 3-month Wikipedia access trace, are available. Requests that can't be served from the Squid cache are sent to load-balancing servers running the Linux Virtual Server software, which in turn pass them to one of the Apache web servers for page rendering from the database. The web servers deliver pages as requested, performing page rendering for all the language editions of Wikipedia. To increase speed further, rendered pages are cached in a distributed memory cache until invalidated, allowing page rendering to be skipped entirely for most common page accesses.

Wikipedia currently runs on dedicated clusters of Linux servers (mainly Ubuntu). As of December 2009, there were 300 in Florida and 44 in Amsterdam. By January 22, 2013, Wikipedia had migrated its primary data centre to an Equinix facility in Ashburn, Virginia.

In accordance with growing amounts of incoming donations exceeding seven digits in 2013 as recently reported, the Foundation has reached a threshold of assets which qualify its consideration under the principles of industrial organization economics to indicate the need for the re-investment of donations into the internal research and development of the Foundation. Two of the recent projects of such internal research and development have been the creation of a Visual Editor and a largely under-utilized "Thank" tab which were developed for the purpose of ameliorating issues of editor attrition, which have met with limited success. The estimates for reinvestment by industrial organisations into internal research and development was studied by Adam Jaffe who recorded that the range of four percent to twenty-five percent annually was to be recommended, with high end technology requiring the higher level of support for internal reinvestment. At the 2013 level of contributions for Wikimedia presently documented as 45 million dollars, the computed budget level recommended by Jaffe and Caballero for reinvestment into internal research and development is between 1.8 million and 11.3 million dollars annually.

According to the Michael Porter five forces analysis framework for industry analysis, Wikipedia and its parent institution Wikimedia are known as "first movers" and "radical innovators" in the services provided and supported by an open-source, on-line encyclopedia. The "five forces" are centred around the issue of "competitive rivalry" within the encyclopaedia industry where Wikipedia is seen as having redefined by its "radical innovation" the parameters of effectiveness applied to conventional encyclopaedia publication. This is the first force of Porter's five forces analysis. The second force is the "threat of new entrants" with competitive services and products possibly arising on the internet or the web. As a "first mover", Wikipedia has largely eluded the emergence of a fast second to challenge its radical innovation and its standing as the central provider of the services which it offers through the World Wide Web. Porter's third force is the "threat of substitute products" and it is too early to identify Google's "Knowledge Graphs" as an effective competitor given the current dependence of "Knowledge Graphs" upon Wikipedia's free access to its open-source services. The fourth force in the Porter five forces analysis is the "bargaining power of consumers" who use the services provided by Wikipedia, which has historically largely been nullified by the Wikipedia founding principle of an open invitation to expand and edit its content expressed in its moniker of being "the encyclopaedia which anyone can edit." The fifth force in the Porter five forces analysis is defined as the "bargaining power of suppliers", presently seen as the open domain of both the global internet as a whole and the resources of public libraries worldwide, and therefore it isn't seen as a limiting factor in the immediate future of the further development of Wikipedia.

Community-produced news publications include the English Wikipedia's The Signpost, founded in 2005 by Michael Snow, an attorney, Wikipedia administrator and former chair of the Wikimedia Foundation board of trustees. It covers news and events from the site, as well as major events from additional Wikimedia projects, such as Wikimedia Commons. Similar publications are the German-language Kurier, and the Portuguese-language Correio da Wikipédia. Other past and present community news publications on English Wikipedia include the "Wikiworld" web comic, the Wikipedia Weekly podcast, and newsletters of specific WikiProjects like The Bugle from WikiProject Military History and the monthly newsletter from The Guild of Copy Editors. There are additionally a number of publications from the Wikimedia Foundation and multilingual publications such as the and This Month in Education

When the project was started in 2001, all text in Wikipedia was covered by the GNU Free Documentation License (GFDL), a copyleft licence permitting the redistribution, creation of derivative works, and commercial use of content while authors retain copyright of their work. The GFDL was created for software manuals that come with free software programmes licenced under the GPL. This made it a poor choice for a general reference work: for example, the GFDL requires the reprints of materials from Wikipedia to come with a full copy of the GFDL text. In December 2002, the Creative Commons license was released: it was specifically designed for creative works in general, not just for software manuals. The licence gained popularity among bloggers and others distributing creative works on the Web. The Wikipedia project sought the switch to the Creative Commons. Because the two licenses, GFDL and Creative Commons, were incompatible, in November 2008, following the request of the project, the Free Software Foundation (FSF) released a new version of the GFDL designed specifically to allow Wikipedia to relicense its content to CC BY-SA by August 1, 2009. (A new version of the GFDL automatically covers Wikipedia contents.) In April 2009, Wikipedia and its sister projects held a community-wide referendum which decided the switch in June 2009.

The handling of media files (e.g. image files) varies across language editions.

Some language editions, such as the English Wikipedia, include non-free image files under fair use doctrine, while the others have opted not to, in part because of the lack of fair use doctrines in their home countries (e.g. in Japanese copyright law). Media files covered by free content licences (e.g. Creative Commons' CC BY-SA) are shared across language editions via Wikimedia Commons repository, a project operated by the Wikimedia Foundation. Wikipedia's accommodation of varying international copyright laws regarding images has led a few to observe that its photographic coverage of topics lags behind the quality of the encyclopaedic text.

The Wikimedia Foundation isn't a licensor of content, but merely a hosting service for the contributors (and licensors) of the Wikipedia.

This position has been successfully defended in court.

Because Wikipedia content is distributed under an open license, anyone can reuse or re-distribute it at no charge.

The content of Wikipedia has been published in a large number of forms, both online and offline, outside of the Wikipedia website.

Obtaining the full contents of Wikipedia for reuse presents challenges, after direct cloning via a web crawler is discouraged. Wikipedia publishes "dumps" of its contents, but these are text-only; as of 2007 there was no dump available of Wikipedia's images.

Several languages of Wikipedia additionally maintain a reference desk, where volunteers answer questions from the general public. According to a study by Pnina Shachaf in the Journal of Documentation, the quality of the Wikipedia reference desk is comparable to a standard library reference desk, with an accuracy of 55%.

Wikipedia's original medium was for users to read and edit content using any standard web browser through a fixed Internet connection. Although Wikipedia content has been accessible through the mobile web after July 2013, The New York Times on February 9, 2014, quoted Erik Moller, deputy director of the Wikimedia Foundation, stating that the transition of internet traffic from desktops to mobile devices was significant and a cause for concern and worry. The article in The New York Times reported the comparison statistics for mobile edits stating that, "Only 20 percent of the readership of the English-language Wikipedia comes via mobile devices, a figure substantially lower than the percentage of mobile traffic for additional media sites, a large number of of which approach 50 percent. And the shift to mobile editing has lagged even more." The New York Times reports that Mr. Moller has assigned "a team of 10 software developers focused on mobile", out of a total of approximately 200 employees working at the Wikimedia Foundation. One principal concern cited by The New York Times for the "worry" is for Wikipedia to effectively address attrition issues with the number of editors which the online encyclopaedia attracts to edit and maintain its content in a mobile access environment.

Bloomberg BusinessWeek reported in July 2014 that Google's Android mobile apps have dominated the largest share of global smartphone shipments for 2013 with 78.6% of market share over their next closest competitor in iOS with 15.2% of the market. At the time of the Tretikov appointment and her posted web interview with Sue Gardner in May 2014, Wikimedia representatives made a technical announcement concerning the number of mobile access systems in the market seeking access to Wikipedia. Directly after the posted web interview, the representatives stated that Wikimedia would be applying an all-inclusive approach to accommodate as a large number of mobile access systems as possible in its efforts for expanding general mobile access, including BlackBerry and the Windows Phone system, making market share a secondary issue. The latest version of the Android app for Wikipedia was released on July 23, 2014, to generally positive reviews, scoring over four of a possible five in a poll of approximately 200,000 users downloading from Google. The latest version for iOS was released on April 3, 2013, to similar reviews.

Access to Wikipedia from mobile phones was possible as early as 2004, through the Wireless Application Protocol (WAP), via the Wapedia service. In June 2007 Wikipedia launched, an official website for wireless devices. In 2009 a newer mobile service was officially released, located at, which caters to more advanced mobile devices such as the iPhone, Android-based devices or WebOS-based devices. Several additional methods of mobile access to Wikipedia have emerged. Many devices and applications optimise or enhance the display of Wikipedia content for mobile devices, while a few additionally incorporate additional features such as use of Wikipedia metadata (See Wikipedia:Metadata), such as geoinformation.

Wikipedia Zero is an initiative of the Wikimedia Foundation to expand the reach of the encyclopaedia to the developing countries.

Andrew Lih and Andrew Brown both maintain editing Wikipedia with smart phones is difficult and this discourages new potential contributors. Several years running the number of Wikipedia editors has been falling and Tom Simonite of MIT Technology Review claims the bureaucratic structure and rules are a factor in this. Simonite alleges a few Wikipedians use the labyrinthine rules and guidelines to dominate others and those editors have a vested interest in keeping the status quo. Lih alleges there's serious disagreement among existing contributors how to resolve this. Lih fears for Wikipedia's long term future while Brown fears problems with Wikipedia will remain and rival encyclopaedias won't replace it.

Wikipedia is extremely popular.

In February 2014, The New York Times reported that Wikipedia is ranked fifth globally among all websites, stating "With 18 billion page views and nearly 500 million unique visitors a month […] Wikipedia trails just Yahoo, Facebook, Microsoft and Google, the largest with 1.2 billion unique visitors."

In addition to logistic growth in the number of its articles, Wikipedia has steadily gained status as a general reference website after its inception in 2001. About fifty percent of search engine traffic to Wikipedia comes from Google, a good portion of which is related to academic research. The number of readers of Wikipedia worldwide reached 365 million at the end of 2009. The Pew Internet and American Life project found that one third of US Internet users consulted Wikipedia. In 2011 Business Insider gave Wikipedia a valuation of $4 billion if it ran advertisements.

According to "Wikipedia Readership Survey 2011", the average age of Wikipedia readers is 36, with a rough parity between genders.

Almost half of Wikipedia readers visit the site more than five times a month, and a similar number of readers specifically look for Wikipedia in search engine results.

About 47 percent of Wikipedia readers don't realise that Wikipedia is a non-profit organization.

Wikipedia's content has additionally been used in academic studies, books, conferences, and court cases.

The Parliament of Canada's website refers to Wikipedia's article on same-sex marriage in the "related links" section of its "further reading" list for the Civil Marriage Act. The encyclopedia's assertions are increasingly used as a source by organisations such as the US federal courts and the World Intellectual Property Organization – though mainly for supporting information rather than information decisive to a case. Content appearing on Wikipedia has additionally been cited as a source and referenced in a few US intelligence agency reports. In December 2008, the scientific journal RNA Biology launched a new section for descriptions of families of RNA molecules and requires authors who contribute to the section to additionally submit a draught article on the RNA family for publication in Wikipedia.

Wikipedia has additionally been used as a source in journalism, often without attribution, and several reporters have been dismissed for plagiarising from Wikipedia.

In 2006, Time magazine recognised Wikipedia's participation (along with YouTube, Reddit, MySpace, and Facebook) in the rapid growth of online collaboration and interaction by millions of people worldwide.

In July 2007 Wikipedia was the focus of a 30-minute documentary on BBC Radio 4 which argued that, with increased usage and awareness, the number of references to Wikipedia in popular culture is such that the word is one of a select band of 21st-century nouns that are so familiar (Google, Facebook, YouTube) that they no longer need explanation.

On September 28, 2007, Italian politician Franco Grillini raised a parliamentary question with the minister of cultural resources and activities about the necessity of freedom of panorama. He said that the lack of such freedom forced Wikipedia, "the seventh most consulted website", to forbid all images of modern Italian buildings and art, and claimed this was hugely damaging to tourist revenues.

On September 16, 2007, The Washington Post reported that Wikipedia had become a focal point in the 2008 US election campaign, saying: "Type a candidate's name into Google, and among the first results is a Wikipedia page, making those entries arguably as important as any ad in defining a candidate. Already, the presidential entries are being edited, dissected and debated countless times each day." An October 2007 Reuters article, titled "Wikipedia page the latest status symbol", reported the recent phenomenon of how having a Wikipedia article vindicates one's notability.

Active participation additionally has an impact.

Law students have been assigned to write Wikipedia articles as an exercise in clear and succinct writing for an uninitiated audience.

A working group led by Peter Stone (formed as a part of the Stanford-based project One Hundred Year Study on Artificial Intelligence

Wikipedia won two major awards in May 2004.

The first was a Golden Nica for Digital Communities of the annual Prix Ars Electronica contest; this came with a €10,000 (£6,588; $12,700) grant and an invitation to present at the PAE Cyberarts Festival in Austria later that year. The second was a Judges'Webby Award for the "community" category. Wikipedia was additionally nominated for a "Best Practices" Webby award.

In 2007, readers of brandchannel.com voted Wikipedia as the fourth-highest brand ranking, receiving fifteen percent of the votes in answer to the question "Which brand had the most impact on our lives in 2006?"

In September 2008, Wikipedia received Quadriga A Mission of Enlightenment award of Werkstatt Deutschland along with Boris Tadić, Eckart Höfling, and Peter Gabriel. The award was presented to Wales by David Weinberger.

In 2015, Wikipedia was awarded both the annual Erasmus Prize, which recognises exceptional contributions to culture, society or social sciences, and the Spanish Princess of Asturias Award on International Cooperation. Speaking at the Asturian Parliament in Oviedo, the city that hosts the awards ceremony, Jimmy Wales praised the work of the Asturian language Wikipedia users. The night of the ceremony, members of the Wikimedia Foundation held a meeting with Wikipedians from all parts of Spain, including the local Asturian community.

Many parodies target Wikipedia's openness and susceptibility to inserted inaccuracies, with characters vandalising or modifying the online encyclopaedia project's articles.

Comedian Stephen Colbert has parodied or referenced Wikipedia on numerous episodes of his show The Colbert Report and coined the related term wikiality , meaning "together we can create a reality that we all agree on—the reality we just agreed on". An Additional example can be found in "Wikipedia Celebrates 750 Years of American Independence", a July 2006 front-page article in The Onion , as well as the 2010 The Onion article "'L.A. Law' Wikipedia Page Viewed 874 Times Today".

In an episode of the television comedy The Office U.S., which aired in April 2007, an incompetent office manager (Michael Scott) is shown relying on a hypothetical Wikipedia article for information on negotiation tactics in order to assist him in negotiating lesser pay for an employee. The tactics he used failed, as a joke about the unreliability of Wikipedia and what anyone can do to change its contents. Viewers of the show tried to add the episode's mention of the page as a section of the actual Wikipedia article on negotiation, but this effort was prevented by additional users on the article's talk page.

"My Number One Doctor]]]]]]", a 2007 episode of the television show Scrubs , played on the perception that Wikipedia is an unreliable reference tool with a scene in which Dr. Perry Cox reacts to a patient who says that a Wikipedia article indicates that the raw food diet reverses the effects of bone cancer by retorting that the same editor who wrote that article additionally wrote the Battlestar Galactica episode guide.

In 2008, the comedic website CollegeHumor

The Dilbert

In July 2009, BBC Radio 4 broadcast a comedy series called Bigipedia

In 2010, comedian Daniel Tosh encouraged viewers of his show, Tosh.0

On August 23, 2013, the New Yorker website published a cartoon with this caption: "Dammit, Manning, have you considered the pronoun war that this is going to start on your Wikipedia page?"

In December 2015, John Julius Norwich stated, in a letter published in The Times

Wikipedia has additionally spawned several sister projects, which are additionally wikis run by the Wikimedia Foundation. These additional Wikimedia projects include Wiktionary, a dictionary project launched in December 2002, Wikiquote, a collection of quotations created a week after Wikimedia launched, Wikibooks, a collection of collaboratively written free textbooks and annotated texts, Wikimedia Commons, a site devoted to free-knowledge multimedia, Wikinews, for citizen journalism, and Wikiversity, a project for the creation of free learning materials and the provision of online learning activities. Of these, only Commons has had success comparable to that of Wikipedia. An Additional sister project of Wikipedia, Wikispecies, is a catalogue of species. In 2012 Wikivoyage, an editable travel guide, and Wikidata, an editable knowledge base, launched.

The most obvious economic effect of Wikipedia has been the death of commercial encyclopedias, especially the printed versions, e.g. Encyclopaedia Britannica, which were unable to compete with a product that's essentially free. Nicholas Carr wrote a 2005 essay, "The amorality of Web 2.0", that criticised websites with user-generated content, like Wikipedia, for possibly leading to professional (and, in his view, superior) content producers' going out of business, because "free trumps quality all the time". Carr wrote: "Implicit in the ecstatic visions of Web 2.0 is the hegemony of the amateur. I for one can't imagine anything more frightening." Others dispute the notion that Wikipedia, or similar efforts, will entirely displace traditional publications. For instance, Chris Anderson, the editor-in-chief of Wired Magazine , wrote in Nature that the "wisdom of crowds" approach of Wikipedia won't displace top scientific journals, with their rigorous peer review process.

There is additionally an ongoing debate about the influence of Wikipedia on the biography publishing business.

"The worry is that, if you can get all that information from Wikipedia, what's left for biography?" said Kathryn Hughes, professor of life writing at UEA and author of The Short Life and Long Times of Mrs Beeton and George Eliot: the Last Victorian.

Wikipedia has seen been widely used as a corpus for linguistic research In computational linguistics, information retrieval and natural language processing. In particular, it commonly serves as a target knowledge base for the entity linking problem, which is then called "wikification", and to the related problem of word sense disambiguation. Methods similar to wikification can in turn be used to find "missing" links in Wikipedia.

In 2015, French researchers Dr José Lages of the University of Franche-Comté in Besançon and Dima Shepelyansky of Paul Sabatier University in Toulouse published a global university ranking based on Wikipedia scholarly citations. They used PageRank"followed by the number of appearances in the 24 different language editions of Wikipedia (descending order) and the century in which they were founded (ascending order)."

A number of interactive multimedia encyclopaedias incorporating entries written by the public existed long before Wikipedia was founded.

The first of these was the 1986 BBC Domesday Project, which included text (entered on BBC Micro computers) and photographs from over 1 million contributors in the UK, and covered the geography, art, and culture of the UK. This was the first interactive multimedia encyclopaedia (and was additionally the first major multimedia document connected through internal links), with the majority of articles being accessible through an interactive map of the UK. The user interface and part of the content of the Domesday Project were emulated on a website until 2008.

Several free-content, collaborative encyclopaedias were created around the same period as Wikipedia (e.g. Everything2), with a large number of later being merged into the project (e.g. GNE). One of the most successful early online encyclopaedias incorporating entries by the public was h2g2, which was created by Douglas Adams in 1999. The h2g2 encyclopaedia is relatively light-hearted, focusing on articles which are both witty and informative.

Subsequent collaborative knowledge websites have drawn inspiration from Wikipedia. Some, such as Susning.nu, Enciclopedia Libre, Hudong, and Baidu Baike likewise employ no formal review process, although a few like Conservapedia aren't as open. Others use more traditional peer review, such as Encyclopedia of Life and the online wiki encyclopaedias Scholarpedia and Citizendium. The latter was started by Sanger in an attempt to create a reliable alternative to Wikipedia. ^[1]

This site, Everipedia, is intended to be the for-profit, modern version of Wikipedia.