Medical Definition of reaction
1: the act or process or an instance of reacting
2: bodily response to or activity aroused by a stimulus:a: an action induced by vital resistance to another action; especially : the response of tissues to a foreign substance (as an antigen or infective agent)b: depression or exhaustion due to excessive exertion or stimulationc: abnormally heightened activity succeeding depression or shockd: a mental or emotional disorder forming an individual's response to his or her life situation
3: the force that a body subjected to the action of a force from another body exerts in the opposite direction
4a: chemical transformation or change : the interaction of chemical entities: the state resulting from such a reactionb: a process involving change in atomic nuclei
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