A book or numbered set of books containing authoritative summary information about a variety of topics in the form of short essays, usually arranged alphabetically by headword or classified in some manner. An entry may be signed or unsigned, with or without illustration or a list of references for further reading. Headwords and text are usually revised periodically for publication in a new edition. In a multivolume encyclopedia, any indexes are usually located at the end of the last volume. Encyclopedias may be general (example: Encyclopedia Americana) or specialized, usually by subject (Encyclopedia of Bad Taste) or discipline (Encyclopedia of Social Work). In electronic publishing, encyclopedias were one of the first formats to include multimedia and interactive elements (example: Grolier Multimedia Encyclopedia Online). The modern encyclopedia began with the 21-volume Encyclopédie edited by Denis Diderot and Jean d'Alembert, an expression of the rationalism of the 18th-century Enlightenment (Cornell University Library). Also spelled encyclopaedia. Synonymous with cyclopedia. Compare with dictionary.

From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm