Agent

A·gent

Agent

a.
  1. Acting; -- opposed to patient, or sustaining, action. [Archaic]

Agent

n.
  1. One who exerts power, or has the power to act; an actor.
    Heaven made us agents, free to good or ill.
  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. a chemical substance having biological effects; a drug. (Biochem., Med.)