chief of staff

  1. the head member of the staff officers of a division or higher unit in the armed forces, or of the Departments of the Army or Air Force
  2. the head of any staff; esp., the head of the physicians and surgeons of a hospital

See chief of staff in American Heritage Dictionary 4

noun pl. chiefs of staff Abbr. C. of S. or CS
  1. The ranking officer of the U.S. Army or Air Force, responsible to the secretary of his or her branch and to the President.
  2. The senior military staff officer at the division level or higher.
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