The senior petty officer; responsible for discipline aboard ship WordNet 3.0 & Open English WordNet

A master-at-arms may be a naval rating, responsible for law enforcement, regulating duties, security, anti-terrorism/force protection for a country's navy; an army officer responsible for physical training; or a member of the crew of a merchant ship responsible for security and law enforcement. Wikipedia

A naval police officer, often addressed as "Master" in the Royal Navy Wikipedia Disambiguation

Military position Wikidata

A warrant officer or chief petty officer responsible for discipline aboard a naval ship. Wiktionary