chief of staff

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The ranking officer of the US Army or Air Force, responsible to the secretary of his or her branch and to the president.
• noun A senior military staff officer at the division level or higher.
• noun The highest-ranking staff member at an institution or organization.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The senior officer of any of several services of the armed forces of several nations.
• noun The head of any of several political departments in several nations.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the senior officer of a service of the armed forces