A trial used by the Clans in Battletech world.
The trial is used to settle disputes, usually those of matters of honor.
Almong the warrior caste, the battle is fought as a duel in Circle of Equals, a defined area of any size. Both duelists use weaponry that doesn't have advantage over other's weaponry in any way - for example, MechWarriors may choose 'Mechs of same tonnage that they're both unfamiliar with.
More information in The Clans: Warriors of Kerensky, FASA 1999, ISBN 1-55560-349-1
Also the name of customizable play against computer in MechWarrior 2. Regrettably, this play mode allows "unfair" games and use of more than one 'Mech per side.