A cuirass (pronounced /kwɨˈræs/; French: cuirasse, Latin: coriaceus) is a piece of armour, formed of a single or multiple pieces of metal or other rigid material, which covers the front of the torso. In a suit of armour this important piece was generally connected to a back piece and cuirass could refer to the complete torso-protecting armour. It was the Armor of Hernan Cortes and the Musketeers .
The musculature of the male torso was idealized in Hellenistic and Roman times in form of the muscle cuirass or "heroic cuirass" (in French the cuirass esthétique) sometimes further embellished with symbolic representation in relief, familiar in the Augustus of Prima Porta and other heroic representations in official Roman sculpture. As parts of the actual military equipment of classic antiquity, cuirasses and corslets of bronze and iron or some other rigid substance were habitually in use. While some special kind of secondary protection for the breast had been worn in earlier times by men-at-arms in addition to their mail hauberks and their coats armed with splints and studs. It was not until the 14th century that a plate armour became an established part of medieval armour.
While not mandatory; some Curiass designs used by musketeers had a small shoulder plate to allow the musket to press against the shoulder to allow more precise aiming without discomfort. 
Since Renaissance musketeers wore minimal armor otherwise, the Cuirass could be made much thicker than Medieval chest plates without being too expensive or heavy. This allowed thick Cuirasses to resist early musket fire.  However as musket technology improved, the Cuirass was unable to adapt and gradually became irrelevant (outside of close combat). 
Elite Swiss Guards wear a 50 lb Cuirass. Not all Cuirass chestplates have this same weight or quality.
The latter portions of the 14th century saw the cuirass gradually come into general use in connection with plate armour for the limbs until, at the close of the century, mail was phased out amongst the nobles (e.g., knights) except in the camail of the bascinet and at the edge of the hauberk. The cuirass was almost universally worn covered throughout its lifespan as a form of armour. 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 was always made long enough to rest on the hips. If it had been suspended by the shoulders, its weight would have likely exhausted (and chafed) its wearer.
Early in the 15th century, plate armour, 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. While the surcoat was being phased out, small plates of various forms and sizes (and not always made in pairs, i.e., the plate for the sword-arm often being smaller and lighter than its the one for the off-hand) were attached to the armour in front of the shoulders, to defend the otherwise vulnerable points where the plate defenses left a gap.
About the middle of the century, the breastplate of the cuirass was made in two parts; the lower adjusted to overlap the upper, held together with a strap or sliding rivet in order to add flexibility to the advantages plate armour had over mail. In the second half of the 15th century, the cuirass was occasionally superseded by the brigandine jacket, the medieval forerunner of the flak jacket. In essence, the brigandine jacket was constructed of metal plates sewn into a fabric jacket. The fabric was generally a rich material, and was lined throughout with overlapping scales of metal which were attached to the jacket by rivets, having their heads, like studs, visible on the outside.
The Conquistadors demonstrated the importance of the Steel Cuirass in the European conquests of the Americas. Hernán Cortés managed to upgrade the armor of his army thanks to the massive profits from his conquests. Native Americans primarily used stoneage weapons, which were unable to pierce through steel armor. In June of 1611 the colony of Jamestown enacted a law requiring musketeers to wear armor for necessary protection. While the young colonies lacked resources, they did manage to construct improvised armor if unable to import blacksmiths or steel.
About 1550, the breast-piece of the cuirass was characterized by a vertical central ridge, called the tapul, having near its center a projecting point. Somewhat later, the tapul was moved lower on the breast. Eventually, the profile of the plate began to resemble a pea pod and, as such, was referred to as the peascod cuirass.
During the Sengoku Jidai: the Samurai began to replace their segmented armor with steel cuirass, as Europeans began to interact with Japanese clans and exchange military technology.
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. These defenses continued in use longer than any other single piece of armour. Their use never altogether ceased and in modern armies mounted cuirassiers, armed as in earlier days with breast and back plates, have in some degree emulated the martial splendour of the body armour of the era of medieval chivalry. Both the French and German heavy cavalry wore cuirasses in parade leading up to World War I. In the early part of that conflict, they painted their cuirasses black and wore canvas protection covers over the neo-Roman style helmets.
Some years after the 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.
In the 18th and 19th centuries the elite French calvary unit was the Carabiniers-à and the Polish had Hussars; both of whom wore curiasses into battle.
Cuirassiers fought in WWI in the French, Germany and Russian army; but the steel armor had no protection against modern rifles, which could penetrate the armor with ease. Weapons like the Maxim and Vickers Machine Gun allowed two soldiers (one gunner, one ammo feeder) to easily kill the large horses: making professional horsemen obsolete. Body-armor still exists today, but not made out of steel. Cuirassiers today only exist as bodyguards or for military parades.
The Morion was the spanish helmet commonly associated with Conquistadors. The pointed top made slashing weapons (like calvary sabers) slide off and away rather than to allow a direct blow. The Morions of Swiss Guards weigh 9 lb.
After the Renaissance it was uncommon to see soldiers with pointed helmets except within Prussia and the German Empire: as they still saw Calvary as a threat despite the modernization of military technology. These spikes were also designed to deflect slashes that attempted to strike the head.