master sergeant

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

• noun A noncommissioned rank in the US Army that is above sergeant first class and below the position of sergeant major.
• noun A noncommissioned rank in the US Air Force that is above technical sergeant and below senior master sergeant.
• noun A noncommissioned rank in the US Marine Corps that is above gunnery sergeant and below the position of sergeant major.
• noun One who holds the rank of master sergeant.

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

• noun military, US A non-commissioned officer in the United States Army, ranking above sergeant first class and below sergeant major; equal in grade and pay to a first sergeant.
• noun military, US A non-commissioned officer in the United States Marine Corps, ranking above gunnery sergeant and below master gunnery sergeant or sergeant major.
• noun military, US A non-commissioned officer in the United States Air Force, ranking above technical sergeant and below senior master sergeant.

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

• noun a senior noncommissioned officer in the Army or Marines