What
"A nontrivial, nearly independent, and replaceable part of a system that fulfills a clear function in the context of a well defined architecture. A component conforms to and provides the physical realization of a set of interfaces.."
The Rational Unified Process Made Easy Per Kroll, Phillipe Krutchen. Addison Wesley.

A minimal software item for which a separate specification is available.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST)