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.
One who gives someone else the power to behave in a certain way.
One who or that which helps something to happen.
One who encourages a bad habit in another (typically drug addiction) by their behaviour.