glass used to be considered a liquid. The most recent "thinking" that Im aware of is , any non crystalline solid that goes through a glass transition on achieving a melting temperature and the "sorta" phase transition (even though its really not a true "Phase rule" phase, it shares all the attributes of a relatively smooth transition between its various phses.
Glass is always amorphous (no crystal structure) and will, upon suitable time begin to puddle. That's why stained glass windows in medieval cathedrals are slightly thicker at the bases of vertical sections than at the tops, and have to be reset every few hundred years or so.
Many organic polymers and plastics are technically "glass" because they are amorphous and go through the glass transition on heating