react

Concise Oxford English Dictionary © 2008 Oxford University Press:
react/riˈakt/

▶verb

• 1 respond to something in a particular way.

  ■ (react against) respond with hostility or a contrary course of action to.

• 2 suffer from adverse physiological effects after ingesting, breathing, or touching a substance.

• 3 Chemistry & Physics interact and undergo a chemical or physical change.

• 4 Stock Exchange (of share prices) fall after rising.

– origin C17: from re- + act, orig. suggested by med. L. react-, reagere ‘do again’.