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