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 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, 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, notably Wikipedia, are electronic and often freely available.