An encyclopedia or encyclopaedia (also spelled encyclopædia , see spelling differences)  is a type of reference work or compendium holding a comprehensive summary of information from either all branches of knowledge or a particular branch of knowledge.  Encyclopedias are divided into articles or entries, which are usually accessed alphabetically by article name.  Encyclopedia entries are longer and more detailed than those in most dictionaries.  Generally speaking, unlike dictionary entries, which focus on linguistic information about words, encyclopedia articles focus on factual information concerning the subject for which the article is named.   
Encyclopedias have existed for around 2,000 years; the oldest still in existence, Naturalis Historia , was written starting in ca. AD 77 by Pliny the Elder and was not fully revised at the time of his death in AD 79. The modern encyclopedia evolved out of dictionaries around the 17th century. Historically, some encyclopedias were contained in one volume, whereas others, such as the Encyclopædia Britannica , the Enciclopedia Italiana (62 volumes, 56,000 pages) or the world's largest, Enciclopedia universal ilustrada europeo-americana (118 volumes, 105,000 pages), became huge multi-volume works. Some modern encyclopedias, such as Everipedia and Wikipedia, are electronic and often freely available.
The word encyclopedia comes from the Koine Greek ἐγκύκλιος παιδεία ,  transliterated enkyklios paideia , meaning "general education" from enkyklios (ἐγκύκλιος), meaning "circular, recurrent, required regularly, general"  and paideia (παιδεία), meaning "education, rearing of a child";  it was reduced to a single word due to an error  by copyists of Latin manuscripts. Together, the phrase literally translates as "complete instruction" or "complete knowledge".
Indeed, the purpose of an encyclopedia is to collect knowledge disseminated around the globe; to set forth its general system to the men with whom we live, and transmit it to those who will come after us, so that the work of preceding centuries will not become useless to the centuries to come; and so that our offspring, becoming better instructed, will at the same time become more virtuous and happy, and that we should not die without having rendered a service to the human race in the future years to come.
Copyists of Latin manuscripts took this phrase to be a single Greek word, enkyklopaidia , with the same meaning, and this spurious Greek word became the New Latin word "encyclopaedia", which in turn came into English. Though the notion of a compendium of knowledge dates back thousands of years, the term was first used in the title of a book in 1517 by Johannes Aventinus: Encyclopedia orbisque doctrinarum, hoc est omnium artium, scientiarum, ipsius philosophiae index ac divisio , and in 1538 by Joachimus Fortius Ringelbergius, Lucubrationes vel potius absolutissima kyklopaideia ( Basel, 1538).
The word encyclopaedia was first used as a noun in the title of his book by the Croatian encyclopedist Pavao Skalić in his Encyclopaedia seu orbis disciplinarum tam sacrarum quam prophanarum epistemon (Encyclopaedia, or Knowledge of the World of Disciplines, Basel, 1559). One of the oldest vernacular uses was by François Rabelais in his Pantagruel in 1532. Several encyclopedias have names that include the suffix -p(a)edia , e.g., Banglapedia (on matters relevant for Bengal).
In British usage, the spellings encyclopedia and encyclopaedia are both current.  In American usage, only the former is commonly used.  The spelling encyclopædia —with the æ ligature —was frequently used in the 19th century and is increasingly rare, although it is retained in product titles such as Encyclopædia Britannica and others. The Oxford English Dictionary (1989) records encyclopædia and encyclopaedia as equal alternatives (in that order), and notes the æ would be obsolete except that it is preserved in works that have Latin titles. Webster's Third New International Dictionary (1997–2002) features encyclopedia as the main headword and encyclopaedia as a minor variant. In addition, cyclopedia and cyclopaedia are now rarely used shortened forms of the word originating in the 17th century.
The modern encyclopedia was developed from the dictionary in the 18th century. Historically, both encyclopedias and dictionaries have been researched and written by well-educated, well-informed content experts, but they are significantly different in structure. A dictionary is a linguistic work which primarily focuses on alphabetical listing of words and their definitions. Synonymous words and those related by the subject matter are to be found scattered around the dictionary, giving no obvious place for in-depth treatment. Thus, a dictionary typically provides limited information, analysis or background for the word defined. While it may offer a definition, it may leave the reader lacking in understanding the meaning, significance or limitations of a term, and how the term relates to a broader field of knowledge. An encyclopedia is, allegedly, not written in order to convince, although one of its goals is indeed to convince its reader about its own veracity. In the terms of Aristotle's Modes of persuasion, a dictionary should persuade the reader through logos (conveying only appropriate emotions); it will be expected to have a lack of pathos (it should not stir up irrelevant emotions), and to have little ethos except that of the dictionary itself.
To address those needs, an encyclopedia article is typically not limited to simple definitions, and is not limited to defining an individual word, but provides a more extensive meaning for a subject or discipline . In addition to defining and listing synonymous terms for the topic, the article is able to treat the topic's more extensive meaning in more depth and convey the most relevant accumulated knowledge on that subject. An encyclopedia article also often includes many maps and illustrations, as well as bibliography and statistics.
Four major elements define an encyclopedia: its subject matter, its scope, its method of organization, and its method of production:
- Encyclopedias can be general, containing articles on topics in every field (the English-language Encyclopædia Britannica and German Brockhaus are well-known examples). General encyclopedias often contain guides on how to do a variety of things, as well as embedded dictionaries and gazetteers. There are also encyclopedias that cover a wide variety of topics but from a particular cultural, ethnic, or national perspective, such as the Great Soviet Encyclopedia or Encyclopaedia Judaica .
- Works of encyclopedic scope aim to convey the important accumulated knowledge for their subject domain, such as an encyclopedia of medicine, philosophy, or law. Works vary in the breadth of material and the depth of discussion, depending on the target audience. (For example, the produced by A.D.A.M., Inc. for the U.S. National Institutes of Health.)
- Some systematic method of organization is essential to making an encyclopedia usable as a work of reference. There have historically been two main methods of organizing printed encyclopedias: the alphabetical method (consisting of a number of separate articles, organized in alphabetical order), or organization by hierarchical categories. The former method is today the most common by far, especially for general works. The fluidity of electronic media, however, allows new possibilities for multiple methods of organization of the same content. Further, electronic media offer previously unimaginable capabilities for search, indexing and cross reference. The epigraph from Horace on the title page of the 18th century Encyclopédie suggests the importance of the structure of an encyclopedia: "What grace may be added to commonplace matters by the power of order and connection."
- As modern multimedia and the information age have evolved, they have had an ever-increasing effect on the collection, verification, summation, and presentation of information of all kinds. Projects such as Everything2, Encarta, h2g2, and Wikipedia are examples of new forms of the encyclopedia as information retrieval becomes simpler. The method of production for an encyclopedia historically has been supported in both for-profit and non-profit contexts. The Great Soviet Encyclopedia mentioned above was entirely state sponsored, while the Britannica was supported as a for-profit institution. By comparison, Wikipedia is supported by volunteers contributing in a non-profit environment under the organization of the Wikimedia Foundation.
Some works entitled "dictionaries" are actually similar to encyclopedias, especially those concerned with a particular field (such as the Dictionary of the Middle Ages , the Dictionary of American Naval Fighting Ships , and Black's Law Dictionary ). The Macquarie Dictionary, Australia's national dictionary, became an encyclopedic dictionary after its first edition in recognition of the use of proper nouns in common communication, and the words derived from such proper nouns.
There are some broad differences between encyclopedias and dictionaries. Most noticeably, encyclopedia articles are longer, fuller and more thorough than entries in most general-purpose dictionaries.  There are differences in content as well. Generally speaking, dictionaries provide linguistic information about words themselves, while encyclopedias focus more on the thing for which those words stand.    Thus, while dictionary entries are inextricably fixed to the word described, encyclopedia articles can be given a different entry name. As such, dictionary entries are not fully translatable into other languages, but encyclopedia articles can be. 
In practice, however, the distinction is not concrete, as there is no clear-cut difference between factual, "encyclopedic" information and linguistic information such as appears in dictionaries. Thus encyclopedias may contain material that is also found in dictionaries, and vice versa. In particular, dictionary entries often contain factual information about the thing named by the word.
Encyclopedias have progressed from the beginning of history in written form, through medieval and modern times in print, and most recently, displayed on computer and distributed via computer networks.
One of the earliest encyclopedic works to have survived to modern times is the Naturalis Historia of Pliny the Elder, a Roman statesman living in the 1st century AD. He compiled a work of 37 chapters covering natural history, architecture, medicine, geography, geology, and all aspects of the world around him. He stated in the preface that he had compiled 20,000 facts from 2000 works by over 200 authors, and added many others from his own experience. The work was published around AD 77-79, although he probably never finished proofing the work before his death in the eruption of Vesuvius in AD 79.
Isidore of Seville, one of the greatest scholars of the early Middle Ages, is widely recognized for writing the first encyclopedia of the Middle Ages, the Etymologiae ( The Etymologies ) or Origines (around 630), in which he compiled a sizable portion of the learning available at his time, both ancient and contemporary. The work has 448 chapters in 20 volumes, and is valuable because of the quotes and fragments of texts by other authors that would have been lost had he not collected them.
The encyclopedia of Suda, a massive 10th-century Byzantine encyclopedia, had 30 000 entries, many drawing from ancient sources that have since been lost, and often derived from medieval Christian compilers. The text was arranged alphabetically with some slight deviations from common vowel order and place in the Greek alphabet.
The early Muslim compilations of knowledge in the Middle Ages included many comprehensive works. Around year 960, the Brethren of Purity of Basra  were engaged in their Encyclopedia of the Brethren of Purity. Notable works include Abu Bakr al-Razi's encyclopedia of science, the Mutazilite Al-Kindi's prolific output of 270 books, and Ibn Sina's medical encyclopedia, which was a standard reference work for centuries. Also notable are works of universal history (or sociology) from Asharites, al-Tabri, al-Masudi, Tabari's History of the Prophets and Kings , Ibn Rustah, al-Athir, and Ibn Khaldun, whose Muqadimmah contains cautions regarding trust in written records that remain wholly applicable today.
The enormous encyclopedic work in China of the Four Great Books of Song , compiled by the 11th century AD during the early Song dynasty (960–1279), was a massive literary undertaking for the time. The last encyclopedia of the four, the Prime Tortoise of the Record Bureau , amounted to 9.4 million Chinese characters in 1000 written volumes.
In the late Middle Ages, several authors had the ambition of compiling the sum of human knowledge in a certain field or overall, for example Bartholomew of England, Vincent of Beauvais, Radulfus Ardens, Sydrac, Brunetto Latini, Giovanni da Sangiminiano, Pierre Bersuire. Some were women, like Hildegard of Bingen and Herrad of Landsberg. The most successful of those publications were the Speculum maius (Great Mirror) of Vincent of Beauvais and the De proprietatibus rerum (On the Properties of Things) by Bartholomew of England. The latter was translated (or adapted) into French, Provençal, Italian, English, Flemish, Anglo-Norman, Spanish and German during the Middle Ages. Both were written in the middle of the 13th century. No medieval encyclopedia bore the title Encyclopaedia – they were often called On nature (De natura, De naturis rerum) , Mirror (Speculum maius, Speculum universale) , Treasure (Trésor) . 
These works were all hand copied and thus rarely available, beyond wealthy patrons or monastic men of learning: they were expensive, and usually written for those extending knowledge rather than those using it.
During the Renaissance the creation of printing allowed a wider diffusion of encyclopedias and every scholar could have his or her own copy. The De expetendis et fugiendis rebus by Giorgio Valla was posthumously printed in 1501 by Aldo Manuzio in Venice. This work followed the traditional scheme of liberal arts. However, Valla added the translation of ancient Greek works on mathematics (firstly by Archimedes), newly discovered and translated. The Margarita Philosophica by Gregor Reisch, printed in 1503, was a complete encyclopedia explaining the seven liberal arts.
The term encyclopaedia was coined by 16th century humanists who misread copies of their texts of Pliny and Quintilian, and combined the two Greek words " enkyklios paideia " into one word, έγκυκλοπαιδεία.  The phrase enkyklios paideia (ἐγκύκλιος παιδεία) was used by Plutarch and the Latin word Encyclopedia came from him.
The first work titled in this way was the Encyclopedia orbisque doctrinarum, hoc est omnium artium, scientiarum, ipsius philosophiae index ac divisio written by Johannes Aventinus in 1517.
The English physician and philosopher, Sir Thomas Browne used the word 'encyclopaedia' in 1646 in the preface to define his Pseudodoxia Epidemica , a major work of the 17th-century scientific revolution. Browne structured his encyclopaedia upon the time-honoured schemata of the Renaissance, the so-called 'scale of creation' which ascends through the mineral, vegetable, animal, human, planetary and cosmological worlds. Pseudodoxia Epidemica was a European best-seller, translated into French, Dutch and German as well as Latin it went through no less than five editions, each revised and augmented, the last edition appearing in 1672.
The beginnings of the modern idea of the general-purpose, widely distributed printed encyclopedia precede the 18th century encyclopedists. However, Chambers' Cyclopaedia, or Universal Dictionary of Arts and Sciences (1728), and the Encyclopédie of Denis Diderot and Jean le Rond d'Alembert (1751 onwards), as well as Encyclopædia Britannica and the Conversations-Lexikon , were the first to realize the form we would recognize today, with a comprehensive scope of topics, discussed in depth and organized in an accessible, systematic method. Chambers, in 1728, followed the earlier lead of John Harris's Lexicon Technicum of 1704 and later editions (see also below); this work was by its title and content "A Universal English Dictionary of Arts and Sciences: Explaining not only the Terms of Art, but the Arts Themselves".
During the 19th and early 20th century, many smaller or less developed languages saw their first encyclopedias, using French, German, and English role models. While encyclopedias in larger languages, having large markets that could support a large editorial staff, churned out new 20-volume works in a few years and new editions with brief intervals, such publication plans often spanned a decade or more in smaller languages.
The second half of the 20th century also saw the proliferation of specialized encyclopedias that compiled topics in specific fields. This trend has continued. Encyclopedias of at least one volume in size now exist for most if not all academic disciplines, including such narrow topics such as bioethics.
By the late 20th century, encyclopedias were being published on CD-ROMs for use with personal computers. Microsoft's Encarta , launched in 1993, was a landmark example as it had no printed equivalent. Articles were supplemented with both video and audio files as well as numerous high-quality images. After sixteen years, Microsoft discontinued the Encarta line of products in 2009. 
In 2001, Jimmy Wales and Larry Sanger launched Wikipedia, a collaboratively edited, multilingual, open-source, free Internet encyclopedia supported by the non-profit Wikimedia Foundation. As of May 22, 2017, there are 5,410, 697 articles in the English Wikipedia. There are 287 different editions of Wikipedia. As of February 2014, it had 18 billion page views and nearly 500 million unique visitors each month.  Wikipedia has more than 25 million accounts, out of which there were over 118,000 active editors globally, as of August 2015. Wikipedia's accuracy was found by a Nature study to be close to that of Encyclopædia Britannica,  with Wikipedia being much larger.
However, critics argue Wikipedia exhibits systemic bias,  and its group dynamics hinder its goals. Many academics, historians, teachers, and journalists reject Wikipedia as a reliable source of information, primarily for being a mixture of truths, half truths, and some falsehoods, and that as a resource about many controversial topics, is notoriously subject to manipulation and spin. 
While Wikipedia is by far the largest web-based encyclopedia, it is not the only one in existence. There are several much smaller, usually more specialized, encyclopedias on various themes, sometimes dedicated to a specific geographic region or time period.