This article is from the Libraries FAQ, by Anthony Wilson email@example.com with numerous contributions by others.
The traditional definition is "a collection of books". The ALA
Glossary of Library and Information Science (Heartsill Young (ed.)
Chicago: ALA, 1983) defines a library as:
"A collection of material organized to provide physical,
bibliographical, and intellectual access to a target group with a
staff that is trained to provide services and programs related to the
information needs of the target group".
Will Manley adds, in his _Manley Art of Librarianship_ (Jefferson, NC:
McFarland, 1993), "An unused collection of books is simply that - an
unused collection of books. It is not a library."