Agent

Part of speech: noun (n.)

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; as, a physical, chemical, or medicinal agent; as, heat is a powerful agent.

Part of speech: adjective (a.)

Actingpatient, or sustaining, action. [Archaic] "The body agent." Bacon.