n One who exerts power, or has the powertoact; an actor.
n One who acts for, or in the place of, another (the principal), by authority from him; one intrusted with the business of another; a substitute; a deputy; a factor.
n 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.
n (computing) In the client-server model, the part of the system that performs information preparation and exchange on behalf of a client or server. Especially in the phrase “intelligent agent” it implies some kind of autonomous process which can communicate with other agents to perform some collective task on behalf of one or more humans.
n (grammar) The participant of a situation that carries out the action in this situation, e.g. "the boy" in the sentences "The boy kicked the ball" and "The ball was kicked by the boy".