Helmet is a protective device used to protect the head. It is the hat worn by soldiers during training and combat, and it is an indispensable tool in people's transportation. It is mostly semi-circular in shape and consists of three main parts: the shell, the lining, and the suspension device. The shell is made of materials such as special steel, glass fiber reinforced plastic, reinforced plastic, leather, and nylon to resist the damage to the head from bullets, fragments, and other impact objects. In ancient China, it was known as the "helm", "head armor", "helmet", or "helmet".
 
Modern helmets are mainly composed of the helmet shell, lining, and suspension systems. Due to different requirements for various activities, there are many types and styles of helmets.
Typically, the shell of a helmet is made from a material with high strength, such as metal, engineering plastic, Kevlar fiber, etc. It deforms to absorb most of the impact force. The lining material has the functions of sweat absorption, insulation, and shock absorption. Helmets often also have the function of further reducing the impact force and preventing the shell fragments from injuring the head. The suspension system is the part between the shell and the lining, which can usually be adjusted to fit different head shapes of different wearers.