first sergeant

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

• noun The senior noncommissioned officer of a US Army or Marine Corps unit, who is equivalent in rank to a master sergeant and performs administrative duties.

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

• noun US In the United States Army, a non-commissioned officer ranking above sergeant first class and below sergeant major; equal in rank to a master sergeant but with greater command authority.
• noun US In the United States Marine Corps, a non-commissioned officer ranking above gunnery sergeant and below sergeant major or master gunnery sergeant; equal in rank to a master sergeant, but with command rather than technical responsibilities.
• noun US In the United States Air Force, an appointment (not a rank) for the senior member of enlisted personnel in a squadron or other unit. The appointment is usually held by a master sergeant.

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

• noun a sergeant in the Army above the rank of staff sergeant and below master sergeant