1. the senior officer of the U.S. Army or Air Force, a member of the Joint Chiefs of Staff, responsible to the secretary of a service branch.
2. (l.c.) the senior or principal staff officer in a brigade or division or higher unit in the U.S. Army or Marine Corps, in a numbered unit in the U.S. Air Force, or in a unit in the U.S. Navy commanded by a rear admiral or one of higher rank.
3. (l.c.) the senior officer in command of a general staff, esp. that of the military forces of a nation.
4. (l.c.) the head of any staff: the chief of staff for the First Lady.