Encyclopedias have progressed from written form in antiquity, to print in modern times. Today they can also be distributed and displayed electronically.

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Ancient times

     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 first century AD. He compiled a work of 37 chapters covering natural history, architecture, medicine, geography, geology, and other 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 Pliny probably never finished editing the work before his death in the eruption of Vesuvius in AD 79.

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Middle Ages

     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 or Origines, in which he compiled a sizable portion of the learning available at his time, both ancient and contemporary.

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     The most popular encyclopedia of the Carolingian Age was the De universo or De rerum naturis by Rabanus Maurus, written about 830.

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     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.

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     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.

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     The 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

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     In late medieval Europe, 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.

 

     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).

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Renaissance

     Medieval encyclopedias were all hand-copied and thus available mostly to 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.

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     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.

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     Financial, commercial, legal, and intellectual factors changed the size of encyclopedias. During the Renaissance, 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. Some publishers would copy another publisher’s work to produce an encyclopedia faster and cheaper so consumers did not have to pay a lot and they would sell more.

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18th–19th centuries

     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".

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     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.

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20th century

     Popular and affordable encyclopedias such as Harmsworth's Universal Encyclopaedia and the Children's Encyclopaedia appeared in the early 1920s.

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     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.

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     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.

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     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.

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21st century

     In 2001, Jimmy Wales and Larry Sanger launched Wikipedia, a multilingual, open-source, free Internet encyclopedia supported by the non-profit Wikimedia Foundation. Unlike commercial online encyclopedias such as Britannica Online, which are written by experts, Wikipedia is collaboratively edited by volunteers. As of 14 March 2018, there are 5,589,157 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. 

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     Many academics, historians, teachers, and journalists reject Wikipedia as a reliable source of information, and Wikipedia is itself not a reliable source according to its own standards. Critics argue that Wikipedia exhibits systemic bias.

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     Wikipedia is by far the largest web-based encyclopedia, but 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. One example is the Stanford Encyclopedia of Philosophy.