Definition: (DOD) The senior or principal member or head of a staff, or the principal assistant in a staff capacity to a person in a command capacity; the head or controlling member of a staff, for purposes of the coordination of its work; a position, that in itself is without inherent power of command by reason of assignment, except that which is invested in such a position by delegation to exercise command in another's name. In the Army and Marine Corps, the title is applied only to the staff on a brigade or division level or higher. In lower units, the corresponding title is executive officer. In the Air Force, the title is applied normally in the staff on an Air Force level and above. In the Navy, the title is applied only on the staff of a commander with rank of commodore or above. The corresponding title on the staff of a commander of rank lower than commodore is chief staff officer, and in the organization of a single ship, executive officer.