It's not glass, but it may be a glass. https://en.m.wikipedia.org/wiki/Glass:

"Scientifically, the term "glass" is often defined in a broader sense, encompassing every solid that possesses a non-crystalline (that is, amorphous) structure at the atomic scale and that exhibits a glass transition when heated towards the liquid state."

Polystyrene is a glass, for example (https://en.m.wikipedia.org/wiki/Glass_transition)