An encyclopedia, also spelled encyclopaedia or encyclopædia is a type of reference work holding a summary of information from either all branches of knowledge or a particular branch of knowledge. Encyclopedias are either 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 over 2,000 years with the oldest still in existence. 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 Encyclopedia universal ilustrada europeo-americana, became huge multi-volume works. Some modern encyclopedias, such as Nupedia and Wikipedia, are electronic and are often freely available.