duel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A prearranged, formal combat between two persons, usually fought to settle a point of honor.
• noun A struggle for domination between two contending persons, groups, or ideas.
• intransitive verb To engage (another) in or as if in formal combat.
• intransitive verb To oppose actively and forcefully.
• intransitive verb To engage in or as if in formal combat.

from The Century Dictionary.

• noun A single combat; specifically, a premeditated and prearranged combat between two persons with deadly weapons, and usually in the presence of at least two witnesses, called seconds, for the purpose of deciding a quarrel, avenging an insult, or clearing the honor of one of the combatants, or of some third party whose cause he champions.
• noun Any fight or contest between two parties; especially, a military contest between parties representing the same arm of the service.

from the GNU version of the Collaborative International Dictionary of English.

• noun A combat between two persons, fought with deadly weapons, by agreement. It usually arises from an injury done or an affront given by one to the other.
• noun (Old Law) a combat between two persons for proving a cause; trial by battel.
• verb obsolete To fight in single combat.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Arranged, regular combat between two private persons, often over a matter of honor.
• noun Historically, the wager of battle (judicial combat)
• noun Any struggle between two contending persons, groups or ideas.
• verb To engage in a duel.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a prearranged fight with deadly weapons by two people (accompanied by seconds) in order to settle a quarrel over a point of honor
• noun any struggle between two skillful opponents (individuals or groups)
• verb fight a duel, as over one's honor or a woman