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 or organisation with a legitimate interest in a given situation, action or enterprise.
A person holding the stakes of bettors, with the responsibility of delivering the pot to the winner of the bet.
A person filing an interpleader action, such as a garnishee or trustee, who acknowledges possession of property that is owed to one or more of several other claimants.
An escrow agent or custodian.