[re.act] vb [NL reactus, pp. of reagere, fr. L re- + agere to act--more at agent] vi (1644) 1: to exert a reciprocal or counteracting force or influence--often used with on or upon
2: to respond to a stimulus
3: to act in opposition to a force or influence--usu. used with against
4: to move or tend in a reverse direction
5: to undergo chemical reaction ~ vt: to cause to react