As practised from the 11th to 20th centuries in Western societies, a duel is an engagement in combat between two individuals, with matched weapons in accordance with their combat doctrines. In the modern application, the term is applied to aerial warfare between fighter pilots. A battle between two warships is also referred to as a duel or a naval duel, especially in the Age of Sail when such encounters were more common.
The Romantic depiction of mediaeval duels was based on either a pretext of defence of honour, usually accompanied by a trusted representative (who might themselves fight, often in contravention of the duelling conventions), or as a matter of challenge of the champion which developed out of the desire of one party (the challenger) to redress a perceived insult to his sovereign's honour. The goal of the honourable duel was often not so much to kill the opponent as to gain "satisfaction", that is, to restore one's honour by demonstrating a willingness to risk one's life for it.
Duels may be distinguished from trials by combat, in that duels were not used to determine guilt or innocence, nor were they official procedures. Indeed, from the early 17th century duels were often illegal in Europe, though in most societies where duelling was socially accepted, participants in a fair duel were not prosecuted, or if they were, were not convicted. Only gentlemen were considered to have honour, and duels were reserved for social equals. Commoners might duel one another occasionally, but if a gentleman's honour were offended by a person of lower class, he would not duel him, but would beat him with a cane, riding crop, a whip or have his servants do so. Formal duelling is now virtually never practiced.