(1):
(n.) One who exerts power, or has the power to act; an actor.
(2):
(n.) One who acts for, or in the place of, another, by authority from him; one intrusted with the business of another; a substitute; a deputy; a factor.
(3):
(n.) An active power or cause; that which has the power to produce an effect; as, a physical, chemical, or medicinal agent; as, heat is a powerful agent.
(4):
(a.) Acting; - opposed to patient, or sustaining, action.