A second-order transition in which a supercooled melt yields, on cooling, a glassy structure. Below the glass-transition temperature the physical properties vary in a manner similar to those of the crystalline phase. Example: Lithium disilicate crystals melt at 1305 K; the melt can be supercooled to the glass-transition temperature at approximately 773 K below which the viscous liquid freezes to a rigid amorphous glass.