Agent

Webster's Dictionary

(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.

Charles Buck Theological Dictionary

That which acts: opposed to patient, or that which is acted upon.