A solid state material that can be switched from one state to another. For example, the recording layer in phase change rewritable CDs and DVDs switches between an unstructured amorphous state that absorbs light and a structured crystalline state that allows light to pass. In phase change memory, the storage cell switches between a state of low resistance to one of very high resistance. Contrast with crystalline semiconductor. See amorphous, amorphous silicon, chalcogenide glass, phase change disc and phase change memory.