Diffusionless transformations, also referred to as displacive transformations, are solid-state changes in the crystal structure that do not rely on the diffusion of atoms over long distances. Instead, they occur due to coordinated shifts in atomic positions, where atoms move by a distance less than the span between neighboring atoms while maintaining their relative arrangement. An illustrative instance of this is the martensitic transformation observed in steel. The term "martensite" was initially used to designate the hard and finely dispersed constituent that forms in rapidly cooled steels. Subsequently, it was discovered that other materials, including non-ferrous alloys and ceramics, can undergo diffusionless transformations as well. As a result, the term "martensite" has taken on a more inclusive meaning to encompass the resulting product of such transformations. With diffusionless transformations, there is some form of cooperative, homogeneous movement that results in a change to the crystal structure during a phase change. These movements are small, usually less than their interatomic distances, and the neighbors of an atom remain close.