Encyclopedia is a collection of information about people, places, events, and things. It may deal with all areas of knowledge or it may confine itself to just one area. A general encyclopedia, such as World Book, includes information on topics in every field of knowledge. Specialized encyclopedias provide more detailed and technical information on specific areas of knowledge, such as art, medicine, or the social sciences.

In ancient times, scholars found that the information they needed was scattered in manuscripts and scrolls in various parts of the world. Some scholars made their own reference works by copying long quotations from the works of other authors. Others copied items of information from a variety of sources. These ancient reference works were the ancestors of the encyclopedia. But they differ from encyclopedias in many ways. Early scholars presented information in any order they chose, and they had few ways to check its accuracy. In addition, they wrote only for themselves or other scholars. Encyclopedia editors, on the other hand, carefully organize their material and demand accuracy. They also present information to a large, diverse audience.

The word encyclopedia comes from the Greek words enkyklios paideia, meaning general or well-rounded education. The word did not come into common use until the 1700s.

A well-planned general encyclopedia presents facts about humanity; about beliefs, ideas, and achievements; about the world people live in; and about the universe of which they are a part. It presents these facts without prejudice, using language that is easy to understand.

An encyclopedia is concerned with the who, what, when, where, how, and why of things. For example, an article on radar tells what radar is and who developed it, as well as when and where. It also describes how radar operates and why it is important in everyday life.

No one person can create a general encyclopedia. Such an enterprise calls for the combined talents of scholars and specialists, of editors and educators, of researchers and librarians, and of artists, mapmakers, and production specialists. It also calls for a large investment of money by the publisher. To keep an encyclopedia abreast of events in all fields of knowledge, the publisher must revise it on a regular basis.