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"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)
A response of an organism or living tissue to a stimulus.
The state resulting from such a response.
A chemical change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.
The response of cells or tissues to an antigen, as in a test for immunization.
A pattern of behavior constituting a mental disorder or personality type.