Real Dictionary

agent
If you refer to someone or something as the agent of a particular effect, you mean that they cause this effect.
countable noun: An agent in the arts world is a person who gets work for an actor or musician, or who sells the work of a writer to publishers.
countable noun: An agent is a person who looks after someone else's business affairs or does business on their behalf.
countable noun: An agent is a person who works for a country's secret service.

agent in American English
a person or thing that performs an action or brings about a certain result, or that is able to do so
noun: a person or business authorized to act on another's behalf

agent in British English
noun: a person who acts on behalf of another person, group, business, government, etc; representative

agent in the Pharmaceutical Industry
noun: A substance that has a particular effect or is used for a particular purpose can be referred to as a particular kind of agent.