The temperature at which an amorphous polymer changes from glassy and stiff to rubbery and compliant.
For an amorphous plastic, Tg is the real service ceiling — stiffness falls off a cliff above it. For a semi-crystalline plastic it matters much less, because the crystalline regions keep the material stiff well past Tg.
This is why nylon 6/6 has a Tg of only 50 °C yet works at 105 °C, and why the melting point is not a service limit either. The number to design against is the deflection temperature under load.
Using Tg as the service limit for a semi-crystalline plastic. It is the limit for amorphous ones; semi-crystalline plastics keep working well above it.
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