An encyclopedia (less frequently spelled encyclopædia or encyclopaedia) is a book, a collection of books, or electronic media that contains comprehensive information regarding all human knowledge or in a particular field, and is used for reference purposes. Famous encyclopedias include Encyclopaedia Britannica Eleventh Edition (1911), Encyclopaedia Britannica, Encarta, World Book, Encyclopedia Americana, and Wikipedia. Many specialized encyclopedias have appeared, such as the Catholic Encyclopedia and Jewish Encyclopedia. Since the 1990s electronic publication has increasingly replaced print. Indeed there has never been a full print version of Encarta or Wikipedia.
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