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.
Etymology: RE- + ACT or med.L reagere react- (as RE-, L agere do, act)