The Ushaan is an Andorian tradition which means "intimate" or "private" combat that was developed in the planet's brutal violent past. It was created by the Andorian reformist Lor'Vela who created this trial of combat to settle the matters of honor through combat between two individuals. The ritual was normally fought to the death.
The Ushaan also covers the body of rules and regulations that govern this trial of combat which had 12,000 amendments made even after its original creation. One of these amendments allowed for the right of substitution should the one being challenged was unable to fight. Another amendment to the code allowed a married combatant to postpone the trial by combat indefinitely, should they have no children. Though the trial originally was ended through the death of one of the combatants, in normal practice it was generally sufficient to disable (not kill) an opponent to fulfill the rules of the Ushaan.
Humans first encountered the Ushaan ritual when Commander Shran of the Andorian Imperial Guard challenged Tellarite ambassador Naarg. At the last minute, Naarg's role was substituted by Captain Jonathan Archer who fought in the ambassador's place. Combat ended when one of Shran's antennae was severed by Archer, thereby satiating Shran's desire for revenge and formally ending the fight. (ENT episode: "United"; Last Unicorn RPG module: The Andorians: Among the Clans)