What is "master sergeant"

n. 1 (context military US English) 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. 2 (context military US English) A non-commissioned officer in the United States Marine Corps, ranking above gunnery sergeant and below master gunnery sergeant or sergeant major. 3 (context military US English) A non-commissioned officer in the United States Air Force, ranking above technical sergeant and below senior master sergeant.

n. a senior noncommissioned officer in the army or air force or marines

A master sergeant is the military rank for a senior non-commissioned officer in some armed forces.