Origins
The English word 'cavalry' entered the language around 1546 from French 'cavalerie,' borrowed from Italian 'cavalleria,' which meant both knighthood and a body of mounted soldiers. The Italian word derives from 'cavaliere' (horseman, knight), from Late Latin 'caballārius' (horseman), from Latin 'caballus.' In classical Latin, the standard word for horse was 'equus' (which gave English 'equine,' 'equestrian,' and 'equerry'), while 'caballus' was the informal, possibly slang term for a work horse or pack horse. In one of the great upsets of linguistic history, 'caballus' completely displaced 'equus' in Vulgar Latin and became the standard word for horse in every Romance language: French 'cheval,' Spanish 'caballo,' Italian 'cavallo,' Portuguese 'cavalo,' Romanian 'cal.'
The origin of Latin 'caballus' is uncertain and much debated. The leading theory connects it to a Celtic source, possibly Gaulish, since the Celts were renowned horse-breeders and their language may have contributed specialized horse vocabulary to Latin. Other scholars have proposed a Thracian, Scythian, or even Semitic origin. What is clear is that 'caballus' was not a native Latin word but a borrowing that entered the language through contact with horse-breeding peoples on the periphery of the Roman world.
The relationship between 'cavalry' and 'chivalry' is etymologically intimate. Both derive from 'caballus,' but through different Romance branches. 'Cavalry' came through Italian 'cavalleria' (maintaining the hard 'c' of Latin). 'Chivalry' came through Old French 'chevalerie' (where the Latin 'c' before 'a' had softened to 'ch,' as regularly happened in French). Both words originally described the same thing — the culture and practice of mounted warriors — but they diverged in English: 'cavalry' became the military term (mounted soldiers as a fighting force) while 'chivalry' became the cultural term (the code of conduct associated with knighthood).
Latin Roots
The military role of cavalry has been central to warfare since horses were first domesticated on the Eurasian steppe around 4000 BCE. Chariot warfare preceded mounted cavalry, but by the first millennium BCE, horseback riders had become the decisive arm on the battlefield. The Scythians, Persians, Macedonians, Romans, Huns, Mongols, and medieval European knights all depended on mounted forces. The stirrup, introduced to Europe around the eighth century, transformed cavalry by giving riders a stable platform from which to fight with lance and sword.
Medieval European cavalry — the knights — represented the elite of feudal society. A knight's horse was his most valuable possession, and the cost of maintaining horse, armor, and weapons meant that cavalry service was restricted to the wealthy. This economic reality cemented the association between mounted warfare and aristocracy, an association that persisted into the modern era. Cavalry regiments in European armies remained socially prestigious long after their tactical importance had diminished.
The decline of horse cavalry began with the rise of effective firearms in the sixteenth century and accelerated through the nineteenth. The Charge of the Light Brigade at Balaclava in 1854 — immortalized by Tennyson — demonstrated both the heroism and the futility of cavalry charging into modern guns. By World War I, horse cavalry was largely obsolete on the Western Front, though it remained useful in the Middle East and Eastern Front. The last significant cavalry charges in European warfare occurred in the early stages of World War II.
Later History
Modern armies retain the term 'cavalry' for units that perform the traditional cavalry roles — reconnaissance, screening, rapid maneuver — using armored vehicles instead of horses. The U.S. Army's 1st Cavalry Division, for example, is an armored formation that has not used horses in combat since the 1940s. The terminology persists because the tactical concept — mobile, aggressive forces that can move quickly and strike decisively — remains valid regardless of whether the mobility comes from horses or engines.
The metaphorical use of 'cavalry' — 'the cavalry is coming,' meaning help is arriving — derives from the classic Western film scenario in which mounted soldiers arrive just in time to rescue besieged settlers. This image has become so embedded in English that 'cavalry' now serves as a general metaphor for any rescuing force, whether military or not.