noun

1. A person who performs, plays a part in a theatrical play or film.

2. One who acts; a doer.

3. One who takes part in a situation.

4. An advocate or proctor in civil courts or causes.

5. One who institutes a suit; plaintiff or complainant.

6. (policy debate) One who enacts a certain policy action.

7. The entity that performs a role (in use case analysis).

8. (grammar) The most agent-like argument of a clause, e.g. 'the torpedo' in "The torpedo sank the boat" and "The torpedo fired".