Someone or something that takes part in some action; a doer, an agent.
The entity that performs a role (in use case analysis).
A person who acts a part in a theatrical play or (later) in film or television; a dramatic performer.
The subject performing the action of a verb.
A person who shows initiative and resourcefulness.
A mechanism (usually one operated by electricity, compressed air, a spring, or an explosive gas), attached to an internal-combustion engine, as on an automobile, and used as a means of starting the engine without cranking it by hand.