noun One who exerts power, or has the power to act.
noun One who acts for, or in the place of, another (the principal), by that person's authority; someone entrusted to act on behalf of or in behalf of another, such as to transact business for them.
He worked as an agent for the government.
noun A person who looks for work for another person and brokers a deal between the hiree and hirer.
noun Someone who works for an intelligence agency: whether an officer or employee thereof or anyone else who agrees to help their efforts (for ideology, for money, as blackmailee, or otherwise).
noun An active power or cause or substance; something (e.g. biological, chemical, thermal, etc.) that has the power to produce an effect.
noun 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 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".
noun A cheat who is assisted by dishonest casino staff.