Carbon fiber (British English: carbon fibre) is a type of fiber that is made of carbon atoms. It is also sometimes called graphite fiber. It has the highest compressive strength of all the reinforcing materials (composites), and it has a high strength to weight ratio and low coefficient of thermal expansion. The density of carbon fiber is also much lower than the density of steel. Carbon fiber is in the form of several thousand long, thin strands of material that is composed of mostly carbon atoms.