Source : Webster's Revised Unabridged Dictionary (1913)

React \Re*act"\, v. i.
   1. To return an impulse or impression; to resist the action
      of another body by an opposite force; as, every body
      reacts on the body that impels it from its natural state.

React \Re*act"\, v. t.
   To act or perform a second time; to do over again; as, to
   react a play; the same scenes were reacted at Rome.

Source : WordNet®

react
     v 1: show a response or a reaction to something [syn: {respond}]
     2: undergo a chemical reaction; react with another substance
        under certain conditions; "The hydrogen and the oxygen
        react"
     3: act against or in opposition to; "She reacts negatively to
        everything I say" [syn: {oppose}]