An encyclopedia (also spelled encyclopaedia or encyclopædia) is a type of reference work – a compendium holding a 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 to cover the thing or concept for which the article name stands.

Encyclopedias have existed for around 2,000 years; the oldest still in existence, Naturalis Historia, was written in ca. AD 77 by Pliny the Elder. The modern encyclopedia evolved out of dictionaries around the 17th century. Historically, some encyclopedias were contained in one volume, but some, such as the Encyclopædia Britannica or the world´s largest Enciclopedia universal ilustrada europeo-americana, became huge multi-volume works. Some modern encyclopedias, such as Wikipedia, are electronic and are often freely available.

The word encyclopaedia comes from the Koine Greek ἐγκυκλοπαιδεία, from Greek ἐγκύκλιος παιδεία, and is transliterated enkyklios paideia, meaning "general education": enkyklios (ἐγκύκλιος), meaning "circular, recurrent, required regularly, general" + paideia (παιδεία), meaning "education, rearing of a child", but 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. —Diderot

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