The metal suit worn by knights as protection in battle is called armor. You can still see suits of armor in many museums today.
These days, soldiers still use armor to protect them from bullets and other weapons, though it's not usually made of metal. Modern armor, like bullet-proof vests, is made of extremely strong, dense fabric, while 13th century armor was typically made of steel. In the 14th century, the word itself meant "defensive covering worn in combat" and "means of protection," from the Old French armeure, "weapons or armor," with its Latin root, arma, "arms or gear."