■ verb
1》 respond to something in a particular way.
↘( ~ 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. ~- , reagere 'do again'.