v. 1 intr. (foll. by to) respond to a stimulus; undergo a change or show behaviour due to some influence (how did they react to the news?). 2 intr. (often foll. by against) be actuated by repulsion to; tend in a reverse or contrary direction. 3 intr. (often foll. by upon) produce a reciprocal or responsive effect; act upon the agent (they react upon each other). 4 intr. (foll. by with) Chem. & Physics (of a substance or particle) be the cause of activity or interaction with another (nitrous oxide reacts with the metal). 5 tr. (foll. by with) Chem. cause (a substance) to react with another. 6 intr. Mil. make a counter-attack. 7 intr. Stock Exch. (of shares) fall after rising.
[ RE- + ACT or med.L reagere react- (as RE-, L agere do, act) ]