- 1: The action or process of contracting: the state of being contracted (contraction of hepatitis, lung expansion and contraction in breathing--P. G. Donohue).
2: The action of a functioning muscle or muscle fiber in which force is generated accompanied especially by shortening and thickening of the muscle or muscle fiber or sometimes by its lengthening (isometric contraction, isotonic contraction); especially: the shortening and thickening of a functioning muscle or muscle fiber.
3: One of usually a series of rhythmic tightening actions of the uterine muscles (as during menstruation or labor).
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.