United States Secretary of Defense

The United States secretary of defense (SecDef), secondarily titled the secretary of war (SecWar), is the head of the United States Department of Defense (DoD), the executive department of the U.S. Armed Forces, and is a high-ranking member of the Cabinet of the United States. The secretary of defense's position of command and authority over the military is second only to that of the president of the United States, who is the commander-in-chief.

Source: Wikipedia — United States Secretary of Defense (CC BY-SA 4.0)