Main Entry: re·ac·tion

Pronunciation: \rē-ˈak-shən\

Function: noun

Date: circa 1611

1 a : the act or process or an instance of reacting b : resistance or opposition to a force, influence, or movement; especially : tendency toward a former and usually outmoded political or social order or policy
2 : a response to some treatment, situation, or stimulus <her stunned reaction to the news>; also : such a response expressed verbally <critical reaction to the play>
3 : 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 : heightened activity and overaction succeeding depression or shock d : a mental or emotional disorder forming an individual's response to his or her life situation
4 : the force that a body subjected to the action of a force from another body exerts in the opposite direction
5 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