reaction Look up reaction at Dictionary.com
1611, from re- "again, anew" + action (q.v.). Modeled on Fr. réaction, older It. reattione, from M.L. reactionem (nom. reactio), from L.L. react-, pp. stem of reagere "react," from re- "back" + agere "to do, act" (see act). 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. The verb react is attested from 1644.