A person who performs, plays a part in a theatrical play or film.
One who acts; a doer.
One who takes part in a situation.
An advocate or proctor in civil courts or causes.
One who institutes a suit; plaintiff or complainant.
(policy debate) One who enacts a certain policy action.
The entity that performs a role (in use case analysis).
(grammar) The most agent-like argument of a clause, e.g. 'the torpedo' in "The torpedo sank the boat" and "The torpedo fired".