reaction

re·ac·tion

noun \rē-ˈak-shən\

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 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