agent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that acts or has the power or authority to act.
• noun One empowered to act for or represent another.
• noun A means by which something is done or caused; an instrument.
• noun A force or substance that causes a change.
• noun A representative or official of a government or administrative department of a government.
• noun A spy.
• noun Linguistics The argument that expresses the means or cause of an action or event described by a phrase or clause. The noun John is the agent in the clause John threw the ball.
• intransitive verb To act as an agent or representative for.
• intransitive verb To act as an agent or representative.

from The Century Dictionary.

• Acting: opposed to patient in the sense of sustaining action.
• noun An active cause; an efficient cause; one who or that which acts or has the power to act: as, a moral agent; many insects are agents of fertilization.
• noun A person acting on behalf of another, called his principal; a representative; a deputy, factor, substitute, or attorney. Often abbreviated to agt.
• noun An official: as, an agent of police.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Archaic Acting; -- opposed to patient, or sustaining, action.
• noun One who exerts power, or has the power to act; an actor.
• noun 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.
• noun An active power or cause; that which has the power to produce an effect, such as a physical, chemical, or medicinal agent.
• noun (Biochem., Med.) a chemical substance having biological effects; a drug.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who exerts power, or has the power to act; an actor.
• noun 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.
• noun 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.
• noun 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.
• noun 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".

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any agent or representative of a federal agency or bureau
• noun a businessman who buys or sells for another in exchange for a commission
• noun an active and efficient cause; capable of producing a certain effect
• noun the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause
• noun a representative who acts on behalf of other persons or organizations
• noun a substance that exerts some force or effect