An Encyclopedia

1. A comprehensive reference work (often spanning several printed volumes) with articles (usually arranged in alphabetical order, or sometimes arranged by category) on a range of subjects, sometimes general, sometimes limited to a particular field.
2. (dated) The circle of arts and sciences; a comprehensive summary of knowledge, or of a branch of knowledge.

Usage notes

The spelling encyclopedia is standard in American English, preferred in Canadian English, accepted in Australian and International English, and also very common in British English. It is more common than encyclopaedia, for example, in UK newspapers on Google News in 2009 by a 7:3 margin