why is glass considered a super cooled liquid???
In case of glasses, due to either the size of the molecules or other factors, the glass effectively becomes trapped in its disordered state as it cools. It is sometimes said to go through a second order thermodynamic transition i.e. there is not an abrupt change in properties such as its energy. It does not become crystalline. It is thus, in a sense, a supercooled liquid since it is below its thermodynamic freezing point and yet has not crystallized.
One can also give this reason that similar to liquids, glass has a tendency to flow very slowly. Therefore, it is considered a super cooled liquid.