Sergeant
In most non-naval military organizations, a sergeant is a non-commissioned officer (NCO) ranking above privates and corporals, and below warrant officers and commissioned officers.
There are usually several ranks of sergeant, each corresponding to greater experience and responsibility for the daily lives of the soldiers of larger units. Although even the highest sergeant is officially lower in rank than the lowest lieutenant, an experienced sergeant will have considerable personal power and know how to exercise it; a wise junior officer will heed the advice of the senior sergeant.

Sergeant insignia
(U.S. Army)
In the U.S. Army, although there are many ranks of sergeant the lowest carries the title Sergeant, referred to as buck sergeant when needed to distinguish it from the higher ranks. Sergeant is the fifth enlisted rank in the U.S. Army, just above Corporal and below Staff Sergeant, and is the lowest form of non-commissioned officer.

Sergeant insignia
(U.S. Marine Corps)
Similarly, the U.S. Marine Corps has many ranks which carry the title of sergeant, the lowest of which is Sergeant. U.S.M.C. Seargeants are the fifth enlisted rank in the U.S. Marine Corps, just above Corporal and below Staff Sergeant.
Other militaries have ranks with similar names and duties, although the details are usually unique to each.
Sergeant is also the name of an urchin in the streets of Rotterdam, in Orson Scott Card's Ender's Shadow. Sergeant is much smarter than he lets on, but not nearly smart enough to get into Battle School.Related articles