A hashtag is a type of metadata tag used on social network of Twitter and microblogging services, allowing users to apply dynamic , user-generated tagging that makes it possible for others to easily find messages with a specific theme or content; it allows easy, informal markup of folk taxonomy without need of any formal taxonomy or markup language .
Users create and use hashtags by placing the number sign or pound sign
# (also known as the hash character ) in front of a string of alphanumeric characters, usually a word or unspaced phrase, in or at the end of a message. The hashtag may contain letters, digits, and underscores.  Searching for that hashtag will yield each message that has been tagged with it. A hashtag archive is consequently collected into a single stream under the same hashtag.  For example, on the photo-sharing service Instagram , the hashtag #bluesky allows users to find all the posts that have been tagged using that hashtag.
Because of its widespread use, hashtag was added to the Oxford English Dictionary in June 2014.  The term hashtag can also refer to the hash symbol itself when used in the context of a hashtag. 
Origin and use
The US pound sign or hash symbol "#" is often used in information technology to highlight a special meaning. ("Pound sign" in the UK means " £ "; "#" is called hash, gate, and occasionally octothorpe .) In 1970, for example, the number sign was used to denote immediate address mode in the assembly language of the PDP-11  when placed next to a symbol or a number. In 1978, Brian Kernighan and Dennis Ritchie used # in the C programming language for special keywords that had to be processed first by the C preprocessor . In the 1986 SGML standard, ISO 8879:1986 (q.v.), # is a reserved name indicator (rni) which precedes keyword syntactic literals, --e.g., the primitive content token #PCDATA, used for parsed character data.
The pound sign was adopted for use within IRC networks circa 1988 to label groups and topics. Channels or topics that are available across an entire IRC network are prefixed with a hash symbol # (as opposed to those local to a server, which use an ampersand '&'). 
The use of the pound sign in IRC inspired  Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network. He posted the first hashtag on Twitter:
How do you feel about using # (pound) for groups. As in #barcamp [msg]?— Chris Messina, ("factoryjoe"), August 23, 2007 
Messina’s suggestion to use the hashtag was not adopted by Twitter, but the practice took off after hashtags were widely used in tweets relating to the 2007 San Diego forest fires in Southern California.  
According to Messina, he suggested use of the hashtag to make it easy for "lay" users to search for content and find specific relevant updates; they were for people who do not have the technological knowledge to navigate the site. Therefore, the hashtag "was created organically by Twitter users as a way to categorize messages."  Today they are for anyone, either with or without technical knowledge, to easily impose enough annotation to be useful without needing a more formal system or adhering to many technical details.
Internationally, the hashtag became a practice of writing style for Twitter posts during the 2009–2010 Iranian election protests ; Twitter users inside and outside Iran used both English- and Persian -language hashtags in communications during the events. 
The first published use of the term "hash tag" was in a blog post by Stowe Boyd, "Hash Tags = Twitter Groupings,"  on August 26, 2007, according to lexicographer Ben Zimmer , chair of the American Dialect Society's New Words Committee.
Beginning July 2, 2009, Twitter began to hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words). In 2010, Twitter introduced " Trending Topics " on the Twitter front page, displaying hashtags that are rapidly becoming popular. Twitter has an algorithm to tackle attempts to spam the trending list and ensure that hashtags trend naturally. 
Although the hashtag started out most popularly on Twitter as the main social media platform for this use, the use has extended to other social media sites including Instagram, Facebook, Flickr, Tumblr, and Google+. 
A hashtag must begin with a hash character followed by other characters, and is terminated by a space, or end of message. It is always safe to precede the “#” with a space, and to include letters without diacritics, digits, and underscores.  In many cases other characters are also allowed, in particular accented characters used in many languages, but handling may vary from one client to another, and from time to time as standards evolve. A discussion of hashtag standards suggests that if #Romeo&Juliet is used, different Twitter clients might link to #Romeo, #Romeo&, or #Romeo&Juliet.  Hashtags are not case sensitive; a search for “#hashtag” will find “#HashTag”. The use of embedded capitals ( CamelCase ) increases readability and avoids confusion; a (real) pen shop would be advised to use #PenIsland rather than all lower-case.  On microblogging and social networking sites hashtags can be inserted anywhere within a text, often at the beginning or the end, but also within the text, usually as a word (e.g. “It is #sunny today”).
Languages which do not use letters are handled slightly differently. In China, microblogs Sina Weibo and Tencent Weibo use a double-hashtag-delimited #HashName# format, since the lack of spacing between Chinese characters necessitates a closing tag. Twitter uses a different syntax for Chinese characters and orthographies with similar spacing conventions: the hashtag contains unspaced characters, separated from preceding and following text by spaces (e.g. '我 #爱 你' instead of '我#爱你')  or by zero-width non-joiner characters before and after the hashtagged element, to retain a linguistically natural appearance (displaying as unspaced '我#爱你', but with invisible non-joiners delimiting the hashtag). 
It is considered acceptable to tag a post once when contributing to a specific conversation. Two hashtags are considered acceptable when adding a location to the conversation. Three hashtags are seen by some as the "absolute maximum", and any contribution exceeding this risks "raising the ire of the community." 
As well as frustrating other users, the misuse of hashtags can lead to account suspensions. Twitter warns that adding hashtags to unrelated tweets, or repeated use of the same hashtag without adding to a conversation, could cause an account to be filtered from search, or suspended. 
Hashtags are mostly used in unmoderated, ad hoc discussion forums; any combination of characters led by a hash symbol is a hashtag, and any hashtag, if promoted by enough individuals, can "trend" and attract more individual users to discussion. On Twitter, when a hashtag becomes extremely popular, it will appear in the "Trending Topics" area of a user's homepage. The trending topics can be organized by geographic area or by all of Twitter. Hashtags are neither registered nor controlled by any one user or group of users. They cannot be "retired" from public usage, meaning that any given hashtag can theoretically be used in perpetuity. They do not contain any set definitions, meaning that a single hashtag can be used for any number of purposes, as chosen by the creators of them.
Hashtags intended for discussion of a particular event tend to use an obscure wording to avoid being caught up with generic conversations on similar subjects, such as a cake festival using #cakefestival rather than simply #cake. However, this can also make it difficult for topics to become "trending topics" because people often use different spelling or words to refer to the same topic. In order for topics to trend, there has to be a consensus, whether silent or stated, that the hashtag refers to that specific topic.
Hashtags also function as beacons in order for users to find and "follow" (subscribe) or "list" (organize into public contact lists) other users of similar interest.
Television broadcasters such as Channel 4 have employed the hashtag during the transmission of programmes such as First Dates and The Undateables . Research has shown that audience numbers go up when individuals can be interactive by tweeting while viewing a programme.
Hashtags can be used on the social network Instagram , by posting a picture and hashtagging it with its subject. As an example, a photo of oneself and a friend posted to the social network can be hashtagged #bffl or #friends. Instagram has banned certain hashtags, some because they are too generic, such as #photography #iPhone #iphoneography, and therefore do not fulfill a purpose. They have also blocked hashtags that can be linked to illegal activities, such as drug use.  The ban against certain hashtags has a consequential role in the way that particular subaltern communities are built and maintained on Instagram. Despite Instagram's content policies, users are finding creative ways of maintaining their practices and ultimately circumventing censorship. 
Hashtags are also used informally to express context around a given message, with no intent to categorize the message for later searching, sharing, or other reasons. One of the functions of the hashtag is to serve as a reflexive meta-commentary, which contributes to the idea of how written communication in new media can be paralleled to how pragmatic methodology is applied to speech.
This can help express contextual cues or offer more depth to the information or message that appears with the hashtag. "My arms are getting darker by the minute. #toomuchfaketan". Another function of the hashtag can be used to express personal feelings and emotions. For example, with "It's Monday!! #excited #sarcasm" in which the adjectives are directly indicating the emotions of the speaker. It can also be used as a disclaimer of the information that the hashtag accompanies, as in, "BREAKING: US GDP growth is back! #kidding". In this case, the hashtag provides an essential piece of information in which the meaning of the utterance is changed entirely by the disclaimer hashtag. This may also be conveyed with #sarcasm, as in the previous example. Self-mockery is another informal function of the hashtag used by writers, as in this tweet: "Feeling great about myself till I met an old friend who now races at the Master's level. Yup, there's today's #lessoninhumility," where the informality of the hashtag provides commentary on the tweet itself. 
The feature has been added to other, non-short-message-oriented services, such as the user comment systems on YouTube and Gawker Media . In the case of the latter, hashtags for blog comments and directly submitted comments were used to maintain a more constant rate of user activity even when paid employees were not logged into the website. Real-time search aggregators such as the former Google Real-Time Search also support hashtags in syndicated posts, meaning that hashtags inserted into Twitter posts can be hyperlinked to incoming posts falling under that same hashtag; this has further enabled a view of the "river" of Twitter posts that can result from search terms or hashtags.
The use of hashtags has extended to television —a concept that began rising in prominence in the early 2010s. Broadcasters may display a hashtag as an on-screen bug , encouraging viewers to participate in a backchannel of discussion via social media prior to, during, or after the program. Television commercials have sometimes contained hashtags for similar purposes. Hashtag bugs appear on either corner of the screen, or they may appear at the end of an advertisement.
While personalities associated with broadcasts, such as hosts and correspondents, also promote their corporate or personal Twitter usernames in order to receive mentions and replies to posts, usage of related or "branded" hashtags alongside Twitter usernames (e.g., #edshow as well as @edshow ) is increasingly encouraged as a microblogging style in order to "trend" the hashtag (and, hence, the discussion topic) in Twitter and other search engines. Broadcasters also make use of such a style in order to index select posts for live broadcast. Chloe Sladden, Twitter's director of media partnerships, identified two types of television-formatted usage of hashtags: hashtags which identify a series being broadcast (i.e. #SunnyFX ) and instantaneous, "temporary" hashtags issued by television personalities to gauge topical responses from viewers during broadcasts. Some have speculated that hashtags might take the place of (or co-exist with) the Nielsen television ratings system .
An example of trending "temporary" hashtags garnering viewers during broadcasts is observed on The Tonight Show with Jimmy Fallon , a variety talk show on NBC . Every Wednesday, Fallon hosts a segment on his show called "Tonight Show Hashtags," which engages viewers by inviting them via Twitter to post humorous stories based on a specific hashtag topic, such as #WhydidIsaythat, #Worstfirstdate, to #Onetimeinclass, reflecting on funny experiences in daily life. By using hashtags, Fallon creates a sense of community and solidarity among his viewers and draws a wider range of viewers through an online platform while they watch a classic, non-interactive television program. Because of its popularity, the "Tonight Show Hashtags" are usually the 'most tweeted hashtag' on Twitter, which promotes the show. By engaging viewers with a lighthearted subject and simple hashtags, Fallon can gauge topical responses from viewers during broadcasts and also use the hashtags to brand his show.
The increased usage of hashtags as brand promotion devices has been compared to the promotion of branded " keywords " by AOL in the late 1990s and early 2000s, as such keywords were also promoted at the end of television commercials and series episodes.
The late-night television comedy game show @midnight with Chris Hardwick on Comedy Central features a daily game entitled "Hashtag Wars," in which three comedians compete against one another to come up with phrases based on a given hashtag theme.
Some hashtags have become famous worldwide. For instance the slogan " Je suis Charlie , " which was first used on Twitter as the hashtag #jesuischarlie and #iamcharlie to indicate solidarity with Charlie Hebdo offices attacked in Paris, spread to the internet at large.
Since February 2013 Twitter and American Express have collaborated to enable users to pay for discounted goods online by tweeting a special hashtag. American Express members can sync their card with Twitter and pay for offers by tweeting; American Express tweets a response to the member that confirms the purchase. 
Organized real-world events have used hashtags and ad hoc lists for discussion and promotion among participants. Hashtags are used as beacons by event participants in order to find each other, both on Twitter and, in many cases, during actual physical events.
Companies and advocacy organizations have taken advantage of hashtag-based discussions for promotion of their products, services or campaigns.
Political protests and campaigns in the early 2010s, such as #OccupyWallStreet and #LibyaFeb17 , have been organized around hashtags or have made extensive usage of hashtags for the promotion of discussion.
Hashtags are often used by consumers on social media platforms in order to complain about the customer service experience with large companies. The term "bashtag" has been created to describe situations in which a user refers to a corporate social media hashtag in order to criticise the company or to tell others about poor customer service. For example, in January 2012, McDonald's created the #McDStories hashtag so that customers could share positive experiences about the restaurant chain. But, the marketing effort was cancelled after two hours when McDonald's received numerous complaint tweets rather than the positive stories they were anticipating. 
The use of hashtags also reveals what feelings or sentiment an author attaches to a statement. This can range from the obvious, where a hashtag directly describes the state of mind, to the less obvious. For example, words in hashtags are the strongest predictor of whether or not a statement is sarcastic  —a difficult AI problem. 
The YouTuber Spencer FC used the hashtag for the name and crest of his YouTube-based association football team, Hashtag United F.C. .
Since the 2012–13 season, the National Basketball Association (NBA) has allowed fans to vote players in as All-Star Game starters on Twitter and Facebook using #NBAVOTE. The tweets and Facebook posts must include #NBAVOTE along with the player's first and last name or Twitter handle. 
In popular culture
During the April 2011 Canadian party leader debate , Jack Layton , then-leader of the New Democratic Party , referred to Conservative Prime Minister Stephen Harper 's crime policies as "a hashtag fail" (presumably #fail).  
The term "hashtag rap ", coined by Kanye West ,  was developed in the 2010s to describe a style of rapping which, according to Rizoh of the Houston Press , uses "three main ingredients: a metaphor, a pause, and a one-word punch line , often placed at the end of a rhyme".  Rappers Nicki Minaj , Big Sean , Drake , and Lil Wayne are credited with the popularization of hashtag rap, while the style has been criticized by Ludacris , The Lonely Island ,  and various music writers. 
In September 2014, in response to the " blame the victim " public reactions to videotaped footage of NFL player Ray Rice assaulting his then-fiancée Janay Palmer in the elevator of an Atlantic City casino, Beverly Gooden shared on Twitter her own story of domestic abuse , using the hashtag #WhyIStayed, and encouraged others to share theirs.  
Hashtags have been used verbally to make a humorous point in informal conversations,  such as "I’m hashtag confused!"  In August 2012, British journalist Tom Meltzer reported in The Guardian about a new hand gesture that mimicked the hashtag, sometimes called the "finger hashtag", in which both hands form a peace sign , and then the fingers are crossed to form the symbol of a hashtag.  The emerging gesture was reported about in Wired by Nimrod Kamer ,  and during 2013, it was seen on TV as used by Jimmy Fallon , and on The Colbert Report , among other programs.  Writing in 2015, Paola Maria Caleff considered this usage a fad , but noted that people talking the way that they write was a consequence of computer-mediated communication. 
- Hashflags: In 2010, Twitter introduced "hashflags" during the 2010 World Cup in South Africa.  They reintroduced the feature on June 10, 2014, in time for the 2014 World Cup in Brazil,   and then again on April 10, 2015, with UK political party logos for the 2015 UK General Election.  When a user tweets a hashtag consisting of the three letter country code of any of the 32 countries represented in the tournament, Twitter automatically embeds a flag emoticon for that country. A similar system was implemented for the 2015 Eurovision Song Contest in Vienna, Austria. 
- Cashtags: In 2009, StockTwits used ticker symbols preceded by the dollar sign to create "cashtags".   In July 2012, Twitter adapted the hashtag style to make company ticker symbols preceded by the dollar sign clickable (as in $AAPL ), a method that Twitter dubbed the "cashtag".   This is intended to allow users to search posts discussing companies and their stocks. This is also used for discussion of currency fluctuations on Twitter, e.g. using #USDGBP or $USDGBP when mentioning the US Dollar's level expressed in Pounds Sterling.