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[ree-ak-shuh n] /riˈæk ʃən/

a reverse movement or tendency; an action in a reverse direction or manner.
movement in the direction of political conservatism or extreme rightism.
action in response to some influence, event, etc.:
the nation’s reaction to the president’s speech.
Physiology. action in response to a stimulus, as of the system or of a nerve, muscle, etc.

Bacteriology, Immunology. the specific cellular response to foreign matter, as in testing for allergies.
Chemistry. the reciprocal action of chemical agents upon each other; chemical change.
Also called nuclear reaction. Physics. a process in which a nucleus that is bombarded by a photon, particle, or other nucleus, emits a nucleon, alpha particle, or the like, without a significant change in its atomic weight.
Mechanics. the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude but opposite in direction to the applied force.
Commerce. a decline in the market after an advance in prices.
a response to some foregoing action or stimulus
the reciprocal action of two things acting together
opposition to change, esp political change, or a desire to return to a former condition or system
a response indicating a person’s feelings or emotional attitude

short for chemical reaction, nuclear reaction
(stock exchange) a sharp fall in price interrupting a general rise

“action in resistance or response to another action or power,” 1610s, from re- “again, anew” + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), noun of action formed in Late Latin from past participle stem of Latin reagere “react,” from re- “back” + agere “to do, act” (see act (v.)).

Originally scientific; physiological sense is attested from 1805; psychological sense first recorded 1887; general sense of “action or feeling in response” (to a statement, event, etc.) is recorded from 1914. Reaction time, “time elapsing between the action of an external stimulus and the giving of a signal in reply,” attested by 1874.

reaction re·ac·tion (rē-āk’shən)


Related Terms

gut reaction


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