Plate armor is body armor that consists of large interlocking plates (called "lames") of iron or steel. It was the ultimate type of armor before, and even partially during, the time of firearms.
Plate armor was known since ancient times, but was first seriously developed in the 14th century CE in Europe. In the 13th century CE small parts of body armor like kneecaps were made of plate, worn over mail, which was the standard heavy armor at the time. Gradually more and more parts started to be equipped with plate: shins, chest, arms, legs and more, until the whole body was covered. During this transition period, plate was often worn over mail or the other way around, making the armor strong but very heavy. By the 15th century CE, full plate had been developed and mail was dispensed with, creating true plate armor. In the Middle East 'plated mail', a mix of plate and mail, remained popular for much longer.
Early plate armor was affordable only by knights and other nobles. Around 1500 CE infantry soldiers were also equipped with it. Their armor was less complete, heavier but of lesser quality and of course less fancy and expensive. This was often called 'munition' armor and actually easier to make and cheaper than mail, partly because labor costs had increased sharply since the Black Death. On the other end of the scale, kings, nobles and knights ended up with 'gothic' plate armor, highly decorated, made to fit the wearer perfectly, very effective and very expensive. These suits were heavier (30 kilograms on average) than 'normal' plate, but as their wearers usually rode into battle on horseback, that was not much of a problem.
Plate armor required the skills of an expert smith to craft and repair and this made it very expensive. But when finished, it offered superb protection against nearly every melee attack. It was so effective because the lames spread the force of a hit over a large surface. The average thickness of the lames varied from 1 mm (on the limbs) through 2 mm (the cuirass) to 3 mm (the helmet). A full suit of plate weighed about 15 - 25 kilograms. Early plate armor was relatively light. It gradually increased in thickness and weight over time, in response to attacks from heavier weapons. As the weight was well-distributed, plate was somewhat easier to wear than other types of armor of the same weight. The story that knights in plate armor had to be hoisted in the saddle using a crane is a fable invented by Mark Twain. Plate-armored warriors actually could run, jump and roll without much difficulty.
The introduction of plate armor provoked a counter-revolution in weaponry. Slashing and cutting swords gave way to piercing weapons like the bastard sword or the extremely specialized estoc. Smashing weapons like maces and warhammers, known since antiquity, regained popularity as plate is less strong against hammering attacks. Among missile weapons, crossbows and firearms became more and more powerful. Despite the heavier weaponry, plate was so effective that soldiers gradually discarded their shields. This allowed them to wield heavy two-handed swords, hammers and polearms, which also packed more punch against enemies in plate armor.
During the 16th century CE, firearms got more numerous and effective. In response smiths developed bullet-proof plate armor, which was extra thick (4 mm) at the front. It was also heavier, up to 40 kilograms. It was mostly worn by cuirassiers, dragoons, demi-lancers and hussars, lance- and pistol-wielding successors of knights on horseback. This was the last adaption that was made to keep up in the arms race. While the heavy cavalry stepped up armor protection, at the same time lighter troops started to dispense with it. By the end of the 17th century CE all their plate armor had disappeared except for the breastplate, which remained common up and into the 18th century CE. Only kings and high commanders kept wearing full suits of armor, not as protection, but as a symbol of authority.