n 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 : esp  :  the response of tissues to a foreign substance (as an antigen or infective agent)  b  :  depression or exhaustion due to excessive exertion or stimulation  c  :  abnormally heightened activity succeeding depression or shock  d  :  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  4 a (1) :  chemical transformation or change :  the interaction of chemical entities (2) :  the state resulting from such a reaction  b  :  a process involving change in atomic nuclei .
 
 
 
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