1a warrant officer in the British army whose job is to assist the adjutant of a regiment or battalion ( regimental sergeant major) or a subunit commander ( company sergeant major, battery sergeant major, etc.).
2a high rank of non-commissioned officer in the US army, above master sergeant and below warrant officer.
3a fish with boldly striped sides which lives in warm seas, typically on coral reefs.
Abudefduf saxatilis, family Pomacentridae
sergeant major in other Oxford dictionaries
Definition of sergeant major in the US English dictionary