In this case the world "plastic" is used in its popular sense. Vast majority of "plastics" are plastic in physical sense. But some are not. Some undergo physic

In this case the world "plastic" is used in its popular sense. Vast majority of "plastics" are plastic in physical sense. But some are not. Some undergo physical or chemical change after they are cooled and are on longer plastic in the physical sense. So maybe not "by definition".