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.
I only use the library for the encyclopedia, as we’ve got most other books here.
My old encyclopedia has been gathering dust on the shelf for the past decade.
Similarly comprehensive works in other formats.
Wikipedia is an online encyclopedia that anyone can edit.
The ancient Mesopotamians never got to write an encyclopedia on stone.
(dated, specifically) The circle of arts and sciences (see Etymology); a comprehensive summary of knowledge, or of a branch thereof.
His life's work is a four-volume encyclopedia of aviation topics.
I have no idea how to build this model railroad, so I'm gonna look it up on Trainpedia, the encyclopedia for all things model train.