Agent

1. Acting; -- opposed to patient, or sustaining, action.
   

1. One who exerts power, or has the power to act; an actor.
2. 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. An active power or cause; that which has the power to produce an effect, such as a physical, chemical, or medicinal agent; as, heat is a powerful agent.
4. (Biochem., Med.) a chemical substance having biological effects; a drug.