The term acting is "…a form of expression in constant use and well understood in all the executive departments of the government as designating, not an appointed incumbent, but merely a locum tenens who is performing the duties of an office to which he does not himself claim title." See Fraser v. United States, 16 Ct. Cl. 507.

Locum tenens is defined as "…holding the place, a deputy, substitute, lieutenant, or representative." See Black's Law Dictionary, 6th Ed., p. 941.