1.NounA 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."
2.NounSimilarly 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."
3.NounThe 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."