The federal department responsible for safeguarding national security; created in 1947; Also called: Defense Department, Defense, DoD.
US government department presided over by the secretary of defense, with headquarters in the Pentagon. The secretary holds a seat in the president's cabinet; each of the three military services has a civilian secretary, not of cabinet rank, at its head. It was established when the army, navy, and air force were unified by the National Security Act 1947.