The
glass–liquid transition or
glass transition for short is the reversible transition in
amorphous materials (or in amorphous regions within
semicrystalline materials) from a hard and relatively brittle "glassy" state into a molten or
rubber-like state, as the temperature is increased. An amorphous solid that exhibits a glass transition is called a
glass. The reverse transition, achieved by
supercooling a
viscous liquid into the glass state, is called
vitrification.