An encyclopedia (American English), encyclopædia, or encyclopaedia (British English) is a reference work or compendium providing summaries of knowledge either from all branches or from a particular field or discipline.[1] Encyclopedias are divided into articles or entries that are often arranged alphabetically by article name[2] and sometimes by thematic categories. Encyclopedia entries are longer and more detailed than those in most dictionaries.[2] Generally speaking, encyclopedia articles focus on factual information concerning the subject named in the article's title; this is unlike dictionary entries, which focus on linguistic information about words, such as their etymology, meaning, pronunciation, use, and grammatical forms.[3][4][5][6]

Encyclopedias have existed for around 2,000 years and have evolved considerably during that time as regards language (written in a major international or a vernacular language), size (few or many volumes), intent (presentation of a global or a limited range of knowledge), cultural perspective (authoritative, ideological, didactic, utilitarian), authorship (qualifications, style), readership (education level, background, interests, capabilities), and the technologies available for their production and distribution (hand-written manuscripts, small or large print runs, Internet). As a valued source of reliable information compiled by experts, printed versions found a prominent place in libraries, schools and other educational institutions.

The appearance of digital and open-source versions in the 21st century, such as Wikipedia, has vastly expanded the accessibility, authorship, readership, and variety of encyclopedia entries.

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.

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. An encyclopedia is, theoretically, not written in order to convince, although one of its goals is indeed to convince its reader of its own veracity.

Four major elements define an encyclopedia: its subject matter, its scope, its method of organization, and its method of production:

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.[2][7] 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.[3][4][5][6] 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.[3]

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.[5][7][8] Thus encyclopedias may contain material that is also found in dictionaries, and vice versa.[8] In particular, dictionary entries often contain factual information about the thing named by the word.[7][8]

As of the early 2020s, the largest encyclopedias are the Chinese Baike.com (18 million articles) and Baidu Baike (16 million), followed by Wikipedias for English (6 million), German (+2 million) and French (+2 million), all of which are wholly online.[9][failed verification] More than a dozen other Wikipedias have 1 million articles or more, of variable quality and length.[9][failed verification] Measuring an encyclopedia's size by its articles is an ambiguous method since the online Chinese encyclopedias cited above allow multiple articles on the same topic, while Wikipedias accept only one single common article per topic but allow automated creation of nearly empty articles.[citation needed]

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, including the Internet.

The earliest encyclopedic work 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. This work became very popular in Antiquity, was one of the first classical manuscripts to be printed in 1470, and has remained popular ever since as a source of information on the Roman world, and especially Roman art, Roman technology and Roman engineering.

The first Christian encyclopedia was the Institutiones divinarum et saecularium litterarum of Cassiodorus (543-560), which were divided in two parts: the first one dealt with Christian Divinity; the second one described the seven liberal arts.

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.

From India, the Siribhoovalaya (Kannada: ಸಿರಿಭೂವಲಯ), dated between 800 A.D to 15th century, is a work of kannada literature written by Kumudendu Muni, a Jain monk. It is unique because rather than employing alphabets, it is composed entirely in Kannada numerals. Many philosophies which existed in the Jain classics are eloquently and skillfully interpreted in the work. The enormous encyclopedic work in China of the Four Great Books of Song, compiled by the 11th century 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.

There were many great encyclopedists throughout Chinese history, including the scientist and statesman Shen Kuo (1031–1095) with his Dream Pool Essays of 1088, the statesman, inventor, and agronomist Wang Zhen (active 1290–1333) with his Nong Shu of 1313, and the written Tiangong Kaiwu of Song Yingxing (1587–1666), the latter of whom was termed the "Diderot of China" by British historian Joseph Needham.[10]

Before the advent of the printing press, encyclopedic 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 [it] 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.

Financial, commercial, legal, and intellectual factors changed the size of encyclopedias. Middle classes had more time to read and encyclopedias helped them to learn more. Publishers wanted to increase their output so some countries like Germany started selling books missing alphabetical sections, to publish faster. Also, publishers could not afford all the resources by themselves, so multiple publishers would come together with their resources to create better encyclopedias. Later, rivalry grew, causing copyright to occur due to weak underdeveloped laws. John Harris is often credited with introducing the now-familiar alphabetic format in 1704 with his English – to give its full title. Organized alphabetically, its content does indeed contain explanation not merely of the terms used in the arts and sciences, but of the arts and sciences themselves. Sir Isaac Newton contributed his only published work on chemistry to the second volume of 1710.

Lexicon Technicum: Or, A Universal English Dictionary of Arts and Sciences: Explaining not only the Terms of Art, but the Arts Themselves

The Encyclopædia Britannica, had a modest beginning in Scotland: the first edition, issued between 1768 and 1771, had just three hastily completed volumes – A–B, C–L, and M–Z – with a total of 2,391 pages. By 1797, when the third edition was completed, it had been expanded to 18 volumes addressing a full range of topics, with articles contributed by a range of authorities on their subjects.

The German-language Conversations-Lexikon was published at Leipzig from 1796 to 1808, in 6 volumes. Paralleling other 18th century encyclopedias, its scope was expanded beyond that of earlier publications, in an effort at comprehensiveness. It was, however, intended not for scholarly use but to provide results of research and discovery in a simple and popular form without extensive detail. This format, a contrast to the Encyclopædia Britannica, was widely imitated by later 19th century encyclopedias in Britain, the United States, France, Spain, Italy and other countries. Of the influential late-18th century and early-19th century encyclopedias, the Conversations-Lexikon is perhaps most similar in form to today's encyclopedias.

The Encyclopædia Britannica appeared in various editions throughout the nineteenth century, and the growth of popular education and the Mechanics' Institutes, spearheaded by the Society for the Diffusion of Useful Knowledge led to the production of the Penny Cyclopaedia, as its title suggests issued in weekly numbers at a penny each like a newspaper.

In the early 20th century, the Encyclopædia Britannica reached its eleventh edition, and inexpensive encyclopedias such as Harmsworth's Universal Encyclopaedia and Everyman's Encyclopaedia were common.

In the United States, the 1950s and 1960s saw the introduction of several large popular encyclopedias, often sold on installment plans. The best known of these were World Book and Funk and Wagnalls. As many as 90% were sold door to door. Jack Lynch says in his book You Could Look It Up that encyclopedia salespeople were so common that they became the butt of jokes. He describes their sales pitch saying, "They were selling not books but a lifestyle, a future, a promise of social mobility." A 1961 World Book ad said, "You are holding your family’s future in your hands right now," while showing a feminine hand holding an order form.[11]

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 video and audio files as well as numerous high-quality images. After sixteen years, Microsoft discontinued the Encarta line of products in 2009.[12] The concept of a free encyclopedia began with the Interpedia proposal on Usenet in 1993, which outlined an Internet-based online encyclopedia to which anyone could submit content and that would be freely accessible. Early projects in this vein included Everything2 and Open Site. In 1999, Richard Stallman proposed the GNUPedia, an online encyclopedia which, similar to the GNU operating system, would be a "generic" resource. The concept was very similar to Interpedia, but more in line with Stallman's GNU philosophy.

It was not until Nupedia and later Wikipedia that a stable free encyclopedia project was able to be established on the Internet.

The English Wikipedia, which was started in 2001, became the world's largest encyclopedia in 2004 at the 300,000 article stage.[13] By late 2005, Wikipedia had produced over two million articles in more than 80 languages with content licensed under the copyleft GNU Free Documentation License. As of August 2009, Wikipedia had over 3 million articles in English and well over 10 million combined in over 250 languages. Wikipedia currently has 6,381,887 articles in English.

Since 2003, other free encyclopedias like the Chinese-language Baidu Baike and Hudong, as well as English language encyclopedias like Citizendium and Knol have appeared. Knol has been discontinued.