A hussar was a member of a class of light cavalry, originally from the Kingdom of Hungary during the 15th and 16th centuries. The title and distinctive dress of these horsemen were subsequently widely adopted by light cavalry regiments in European armies during the late 17th and 18th centuries. By the 19th century, hussars were wearing jackets decorated with braid plus shako or busby fur hats and had developed a romanticized image of being dashing and adventurous.

Several modern armies retain the designation of hussars for armored (tank) units. In addition, a number of mounted units survive which wear historical hussar uniforms on parade or while providing ceremonial escorts.

Etymologists are divided over the derivation of the word hussar. The term came into use in relation to the cavalry of late medieval Hungary, popularized under Matthias Corvinus, made up of Serb warriors. The first written use of the word hussarones (in Latin, plural; in Hungarian: huszár) is found in documents dating from 1432 in Southern Hungary (at the time an area of the Kingdom of Hungary bordering the Ottoman Empire). Two main theories exist:

The hussars reportedly originated in bands of Serb warriors, crossing into southern Hungary after the Ottoman conquest of Serbia in the late 14th century. Regent-Governor John Hunyadi created mounted units inspired by the Ottomans. His son, Matthias Corvinus, later king of Hungary, is claimed to be the creator of these troops, commonly called Rác^{[citation needed]} (a Hungarian exonym for Serbs). Initially, they fought in small bands, but were reorganised into larger, trained formations during the reign of King Matthias Corvinus.

The medieval Hungarian written sources spoke disparagingly and contemptuously of the light cavalry and the hussars in general, and during battles the texts praised only the virtues, endurance, courage, training and achievements of the knights, since during the Middle Ages the Hungarian noble origin soldiers served exclusively as heavy armoured cavalry.

The first hussar regiments comprised the light cavalry of the Black Army of Hungary. Under Corvinus's command, the hussars took part in the war against the Ottoman Empire in 1485 and proved successful against the sipahis (Ottoman cavalry) as well as against the Bohemians and Poles. After the king's death, in 1490, hussars became the standard form of cavalry in Hungary in addition to the heavy cavalry. Their role was limited to irregular warfare, raiding, securing, covering and reconnaissance of main regular forces. According to Antonio Bonfini, this lightly armed cavalry (expeditissimus equitatus) was not allowed to be part of the regular army, when the order of the battle was formed, but was placed outside it in quite separate groups and used to destroy, burn, kill and instil fear in the civilian population, while they rode ahead of the regular army.

The 16th and 17th centuries saw a major change and during the Thirty Years' War they fought as light cavalry and increasingly used firearms, instead of more traditional weapons such as bows and spears. The Habsburg emperors hired Hungarian hussars as mercenaries to serve against the Ottomans and on various battlefields throughout Western Europe.

Early hussars wore armor when they could afford it, as did the later Polish hussars. Hungarian hussars abandoned the use of shields and, at a later date, armor; becoming entirely light cavalry in the first half of the 17th century.