Medical Definition of contraction
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)
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