What is Single Combat?

Legal Definition
Single combat is a duel between two single warriors which takes place in the context of a battle between two armies. Often, it is champion warfare, with the two considered the champions of their respective sides. Instances of single combat are known from Classical Antiquity and the Middle Ages. The champions were often combatants who represented larger, spectator groups. Such representative contests and stories thereof are known worldwide.

Typically, it takes place in the no-man's-land between the opposing armies, with other warriors watching and themselves refraining from fighting until one of the two single combatants has won.

But single combat could also take place within a larger battle. Both ancient and medieval warfare did not always rely on the line or phalanx formation. The Iliad notably describes the battles of the Trojan war as a series of single encounters on the field, and the medieval code of chivalry, partly inspired by this, encouraged the single combat between individual knights on the battlefield, in which the loser was not usually killed but taken captive for ransom. This tradition ended in the 14th century due to the use of the longbow and the pike square against mounted knights (Battle of Crécy, Battle of Laupen), and the tradition of single combat was continued away from the battlefield, with the pas d'armes and the early modern duel.
-- Wikipedia
Legal Definition
A duel; a battle.
-- Ballentine's Law Dictionary