first sergeant
noun
Definitions
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.
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.
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.