Cuirass (French cuirasse, Latin coriaceus, made of leather, from corium, the original breastplate being of leather), the plate armour, whether formed of a single piece of metal or other rigid material or composed of two or more pieces, which covers the front of the wearer's person. In a suit of armour, however, since this important piece was generally worn in connection with a corresponding defence for the back, the term cuirass commonly is understood to imply the complete body-armour, including both the breast and the back plates. Thus this complete body armour appears in the middle ages frequently to have been described as a pair of plates. The corslet (Fr. corselet, diminutive of the Fr. cors, body), a comparatively light cuirass, is more strictly a breast-plate only.

As parts of the military equipment of classic antiquity, cuirasses and corslets of bronze, and at later periods also of iron or some other rigid substance, were habitually in use; but while some special kind of secondary protection for the breast had been worn in earlier times by the men-at-arms in addition to their mail hauberks and their cotes armed with splints and studs, it was not till the 14th century that a regular body-defence of plate can be said to have become an established component of medieval armour.

As this century continued to advance, the cuirass is found gradually to have come into general use, in connection with plate defences for the limbs, until, at the close of the century, the long familiar inter-linked chain-mail is no longer visible in knightly figures, except in the camail of the bassinet and at the edge of the hauberk. The prevailing, and indeed almost the universal, usage throughout this century was that the cuirass was worn covered. Thus, the globose form of the breast-armour of the Black Prince, in his effigy in Canterbury Cathedral, 1376, intimates that a cuirass as well as a hauberk is to be considered to have been covered by the royalty-emblazoned jupon of the prince.

The cuirass, thus worn in the 14th century, was always made of sufficient length to rest on the hips; otherwise, if not thus supported, it must have been suspended from the shoulders, in which case it would have effectually interfered with the free and vigorous action of the wearer.

Early in the 15th century, the entire panoply of plate, including the cuirass, began to be worn without any surcoat; but in the concluding quarter of the century the short surcoat, with full short sleeves, known as the tabard, was in general use over the armour. At the same time that the disuse of the surcoat became general, small plates of various forms and sizes (and not always made in pairs, the plate for the right or sword-arm often being smaller and lighter than its companion) were attached to the armour in front of the shoulders, to defend the otherwise vulnerable points where the plate defences of the upper-arms and the cuirass left a gap on each side.

About the middle of the century, instead of being formed of a single plate, the breast-plate of the cuirass was made in two parts, the lower adjusted to overlap the upper, and contrived by means of a strap or sliding rivet to give flexibility to this defence. In the second half of the 15th century the cuirass occasionally was superseded by the brigandine jacket, a defence formed of some textile fabric, generally of rich material, lined throughout with overlapping scales (resembling the earlier imbricated form) of metal, which were attached to the jacket by rivets, having their heads, like studs, visible on the outside.

In the 16th century, when occasionally, and by personages of exalted rank, splendid surcoats were worn over the armour, the cuirassits breastpiece during the first half of the century, globular in form was constantly reinforced by strong additional plates attached to it by rivets or screws.

About 1550 the breast-piece of the cuirass was characterized by a vertical central ridge, called the tapul, having near its centre a projecting point; this projection, somewhat later, was brought lower down, and eventually the profile of the plate, the projection having been carried to its base, assumed the singular form which led to this fashion of the cuirass being distinguished as the peascod cuirass.

Corslets provided with both breast and back pieces were worn by foot-soldiers in the 17th century, while their mounted comrades were equipped in heavier and stronger cuirasses; and these defences continued in use after the other pieces of armour, one by one, had gradually been laid aside. Their use, however, never altogether ceased, and in modern armies mounted cuirassiers, armed as in earlier days with breast and back plates, have insome degree emulated the martial splendour of the body-armour of the era of medieval chivalry.

Some years after Waterloo certain historical cuirasses were taken from their repose in the Tower of London, and adapted for service by the Life Guards and the Horse Guards. For parade purposes, the Prussian Gardes du Corps and other corps wear cuirasses of richly decorated leather.