noun

• (obsolete, law) Someone who institutes a legal suit; a plaintiff or complainant.
• (obsolete) Someone acting on behalf of someone else; a guardian.
• Someone or something that takes part in some action; a doer, an agent.
• A person who acts a part in a theatrical play or (later) in film or television; a dramatic performer.
• (obsolete, Ancient Rome) An advocate or proctor in civil courts or causes.
• (grammar) The subject performing the action of a verb.
• (software engineering) The entity that performs a role (in use case analysis).