Definitions
1.NounSomeone who institutes a legal suit; a plaintiff or complainant. .mw-parser-output .defdate{font-size:smaller}
2.NounSomeone acting on behalf of someone else; a guardian.
3.NounSomeone or something that takes part in some action; a doer, an agent.
4.NounA person who acts a part in a theatrical play or (later) in film or television; a dramatic performer.
5.NounAn advocate or proctor in civil courts or causes.
6.NounThe subject performing the action of a verb.
7.NounThe entity that performs a role (in use case analysis).