Agent, 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.
1 other Webster entry under this spelling
Agent, a.
Actingpatient, or sustaining, action. [Archaic] "The body agent." Bacon.
Shades of meaning
In strict philosophical usage, the prime mover or doer of an act is the agent. Thus we speak of man as a voluntary agent, a free agent. But in common usage, especially in business, an agent is not the prime actor, but only an instrument or factor, acting under orders or instructions. Compare cause.
Prepositions
An agent of the company for selling, etc.