A sergeant is a high-ranking soldier who's in charge of a group. They often lead teams or platoons.
When we talk about sergeants, it's usually in the military context. You'll hear people talking about army sergeants or police sergeants. A sergeant might be in charge of training new recruits, leading a team on a mission, or even working as an instructor at a military school. Sometimes you'll see the word "sergeant" used as a title in law enforcement too - a police department might have a sergeant who's in charge of a squad or a precinct.
How common is "sergeant"?
Word sergeant is considered uncommon in modern English. It appears most frequently in fiction texts and less often in other writings.
Definitions
noun
(military) A UK army rank with NATO code OR-6, senior to corporal and junior to warrant officer ranks.
The highest rank of noncommissioned officer in some non-naval military forces and police.
(law, historical) A lawyer of the highest rank, equivalent to the doctor of civil law.
(UK, historical) A title sometimes given to the servants of the sovereign.
Example: sergeant surgeon, i.e. a servant, or attendant, surgeon
A bailiff.
A servant in monastic offices.
A fish, the píntano (Abudefduf saxatilis), a species of damselfish.
Any of various nymphalid butterflies of the genus Athyma; distinct from the false sergeants.