Tg

Glass transition or vitrification refer to the transformation of a glass-forming liquid into a glass, which usually occurs upon rapid cooling. It is a dynamic phenomenon occurring between two distinct states of matter (liquid and glass), each with different physical properties. Upon cooling through the temperature range of glass transition (a "glass transformation range"), without forming any long-range order or significant symmetry of atomic arrangement, the liquid contracts more continuously at about the same rate as above the melting point until there is a decrease in the thermal expansion coefficient (TEC). [1]
The glass transition temperature Tg is lower than melting temperature, Tm, due to supercooling. It depends on the time scale of observation which must be defined by convention. One approach is to agree on a standard cooling rate of 10 K/min. Another approach is by requiring a viscosity of 1013 poise. Otherwise, one can only talk about a glass transformation range.

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