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 policy2: 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 direction5 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