Agent

An agent is a person who acts on behalf of another person, or a representative of an agency.

an FBI agent is an official representative of the Federal Bureau of Investigation.
 literary agent, a person who represents a writer
 sports agent, a professional who represents an athlete
 talent agent, a person who finds jobs for actors, models, and other people in various entertainment businesses.
 travel agent, a person who helps plan a trip and buys airline tickets and makes hotel reservations.

Agent can also mean:

 a secret agent, a spy (espionage)
 a free agent, a sports player who is out of contract
 a realtor (US), or real estate agent (Brit.)

In science:

 Agent Orange, a herbicide
 biological agent, an infectious disease that can be used in biological warfare

In fiction:

 Agents, characters in The Matrix
 Secret Agent, a 1936 film