- the action caused by the resistance to another action.
- a return to the opposite physical condition, as after shock, exhaustion, or chill.
Origin of reaction
- any effect produced by the action of a drug, esp an adverse effectCompare side effect
- any effect produced by a substance (allergen) to which a person is allergic, the simultaneous equal and opposite force that acts on a body whenever it exerts a force on another body
"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.