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.
(lowercase) 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.
(lowercase) the senior officer in command of a general staff, especially that of the military forces of a nation.
4.
(lowercase) the head of any staff:
the chief of staff for the First Lady.
Origin
1880-85
British Dictionary definitions for Chief of Staff
Chief of Staff
noun
1.
the senior staff officer under the commander of a major military formation or organization
2.
the senior officer of each service of the armed forces