What is the meaning of Master_sergeant?

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.

A non-commissioned officer in the United States Marine Corps, ranking above gunnery sergeant and below master gunnery sergeant or sergeant major.

A non-commissioned officer in the United States Air Force, ranking above technical sergeant and below senior master sergeant.

Source: wiktionary.org