Webster's Revised Unabridged Dictionary · 1913

Agent

Agent ({not transcribed}) , adjective

[Latin agens, agentis, present participle of agere to act; akin to Greek {not transcribed} to lead, Icelandic aka to drive, Sanskrit aj. r2.]

Acting; -- opposed to patient, or sustaining, action. [Archaic]
The body agent. — Bacon

Agent , noun

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