(1):

(v. t.) To act or perform a second time; to do over again; as, to react a play; the same scenes were reacted at Rome.

(2):

(v. i.) To return an impulse or impression; to resist the action of another body by an opposite force; as, every body reacts on the body that impels it from its natural state.

(3):

(v. i.) To act upon each other; to exercise a reciprocal or a reverse effect, as two or more chemical agents; to act in opposition.