Definition of: react

(rē·akt′) v.i.
1. To act in response, as to a stimulus.
2. To act in a manner contrary to some preceding act; come into or tend toward a former state or an opposite state.
3. Physics To exert an opposite and equal force on an acting or impinging body: said of the body acted upon.
4. Chem. To exert mutual action, as substances undergoing chemical change. Compare RE-ACT.