An encyclopedia or encyclopaedia is a book or series of books that contain general information about many topics or areas. The world encyclopedia comes from the Greek enkyklia paideia, which means "a general knowledge." The word has been in use for at least 500 years, and used in print for the first time in Encyclopaedia, or Knowledge of the World of Disciplines, which was published in 1559.

The first encyclopedia per se was written in the first century BC by Pliny the Elder with the help of his nephew. The work consisted of 37 volumes and covered everything from anthropology and human physiology to agriculture, painting, and pharmacology. Through the following few centuries, encyclopedias adopted a religious overtone. The first Christian encyclopedia was published in 560 BC, and the first Muslim encyclopedia came to light soon after. One of the longest encyclopedias to ever see the light is also one of the oldest. In 1403, the Yongle Encyclopedia was published in China. It consisted of 11,000 handwritten volumes; most of the original work has been lost through the centuries, and less than 400 volumes survive today.

An encyclopedia is often confused with a dictionary, although they are inherently different. A dictionary offers definition of words, while an encyclopedia explores topics more in depth and often includes illustrations, maps, and photographs. While most encyclopedias are organized alphabetically, some are organized by categories or cross-referenced articles.

In the 20th century, the Encyclopedia Britannica has become the standard for encyclopedia works. Topical encyclopedias have also become more popular, and cover topics as varied as Economics, bioethics, or Judaica. Many encyclopedias are now being published in CD-ROM form, with the most notable example being Microsoft's Encarta™; other encyclopedias are now entirely available online, with no print version. One of the most important advantages of online encyclopedias is the fact that they can be edited frequently, so they remain up to date. Traditionally, an encyclopedia was written by a single writer or as a collaborative effort, but new works are being put together by a team of writers that often have no face to face contact whatsoever with each other.