Short form of define, and of definition. A named abbreviation rather than a concept.
Latin definire (to bound, limit, define), from de- (thoroughly) + finire (to bound, finish), from finis (boundary, end); clipped in code and speech to def, a shortened stand-in for define or definition.
Short form of define, and of definition. A named abbreviation rather than a concept of its own, standing in for the drawing of a boundary around a meaning.