agent

noun

• 1a person who acts on behalf of another, in particular:
• 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 travel agent
• a person who works secretly to obtain information for a government or other official body:a trained intelligence agent

• 2a person or thing that takes an active role or produces a specified effect:agents of environmental change bleaching agents
• Grammar the doer of an action, typically expressed as the subject of an active verb or in a by phrase with a passive verb.
• Computing 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.