Actor

Actor, a doer, generally a plaintiff or complainant. In a civil or private action the plaintiff was called by the Romans petitor; in a public action (causa publica) he was called accusator. (Cic. Ad. Att. 1. 16.) The defendant was called reus, both in private and public causes; this term, however, according to Cicero (de Orat. Ii. 43), might signify either party, as indeed we might conclude from the word itself. In a private action the defendant was often called adversarius, but either party might be called so with respect to the other. Also a proctor or advocate in civil Courts or causes. Actor dominicus, a term often used for the lord's bailiff or attorney. Actor ecclesi' was sometimes the forensic term for the advocate or pleading patron of a church. Actor vill' was the steward or head bailiff of a town or village, Cowel.

Actor is one who acts; a person whose conduct is in question, Black Law Dictionary, 7th Edn., p. 35.