A simple, relatively light form of armor, chain mail was one of the main forms of defense during the Middle Ages. As opposed to the heavier, more protective plate armor, chain mail consists of various interlocking links throughout. Chain mail was never designed to completely protect the wearer, but instead helped to make a glancing blow non-fatal. Chain mail became obsolete after the construction of firearms, which made the armor entirely useless.
Properly speaking, it should be referred to as either "Chain" or "Maille". "Chainmail" is a neologism created by Victorian historians.