Webster's English Dictionary

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react

re.act \re--'akt\ vb [NL reactus, pp. of reagere, fr. L re- + agere to act 
   -m more at AGENT 1: to exert a reciprocal or counteracting force or 
   influence - often use d with on or upon 2: to respond to a stimulus  3: to 
   act in opposition to a force or influence - usu. used with agai nst 4: to 
   move or tend in a reverse direction  5: to undergo chemical reaction  : to 
   cause to react