Master chief petty officer
From Halopedia, the Halo wiki
Master Chief Petty Officer is the ninth, and highest enlisted rank of the UNSC Navy, and is a non-commissioned officer. It is equal in rank to Master Gunnery Sergeant and Sergeant Major in the UNSC Marine Corps.
In informal settings, Master Chief Petty Officers are typically called "Master Chief" or "Chief".
The title character of the Halo series, "Master Chief", is named for his rank of Master Chief Petty Officer.
