agent

One who exerts power, or has the power to act.

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.

A person who looks for work for another person and brokers a deal between the hiree and hirer.

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).

An active power or cause or substance; something (e.g. biological, chemical, thermal, etc.) that has the power to produce an effect.

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.

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".

A cheat who is assisted by dishonest casino staff.

A law enforcement officer tasked with enforcing a specific field of law.

A respectful term of address for an agent, especially a law enforcement agent.