Main Entry: re·ac·tion

Pronunciation: \rē-ˈak-shən\

Function: noun

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