An encyclopedia is a compendium, holding a summary of information. This information, taken as a whole, comprises a large reference work from all the branches of knowledge, if it is a general encyclopedia, and information pertaining to a particular area or branch of knowledge if it is a specialized encyclopedia.
Encyclopedias generally are comprised of multiple volumes, divided into articles that have been alphabetized. Without question, the entries in an encyclopedia are much longer and more detailed than those explanations found in a dictionary. That is because dictionaries focus on the linguistic information about words, whereas the articles or entries in an encyclopedia summarize factual information about an object or concept.
Encyclopedias have been in existence for about 2,000 years, and until the invention of home computers and the Internet, their existence as hardbound books was not threatened. While it is true that most school libraries and public libraries still stock at least one set of encyclopedias, it is assumed that the Internet will someday completely supplant them.
Just as e-books are becoming more popular, in part because of ease of use, the hard copies of encyclopedia sets are destined for obsolescence. Unfortunately, buying and storing a multi-volume encyclopedia set is no longer practical, especially when electronic versions on the Internet are freely available.
One of the earliest encyclopedic works is the Naturalis Historia, written and compiled in the 1st century AD by Pliny the Elder. He was a Roman statesman that compiled a work of 37 chapters covering natural history, art and architecture, medicine, geography, geology and all aspects of the world around him. He stated in the preface that he had compiled 80,000 references, and that "It is a difficult task to give novelty to what is old, authority to what is new, brilliance to the common-place, light to the obscure, attraction to the stale, credibility to the doubtful, but nature to all things and all her properties to nature."
The word encyclopaedia comes from the Greek and means general education, or quite literally, "complete instruction" or "complete knowledge."