a person who acts on behalf of another person or group
a person who manages business, financial, or contractual matters for an actor, performer, writer, etc.
a person or company that provides a particular service, typically one that involves organizing transactions between two other parties
a person who works secretly to obtain information for a government or other official body
a person or thing that takes an active role or produces a specified effect
a substance that brings about a chemical or physical effect or causes a chemical reaction
the doer of an action, typically expressed as the subject of an active verb or in a by phrase with a passive verb.
an independently operating Internet program, typically one that performs background tasks such as information retrieval or processing on behalf of a user or other program.