Vitrification is the process of turning something into glass, or into something with a glassy, non-porous surface — and it is also the name for the result of that process. It is the noun that goes with the verb 'vitrify'.

Where you meet it: in pottery and tiles, vitrification is what happens in a hot kiln when the clay body partly melts and fuses, so the finished tile no longer soaks up water — makers of floor tiles advertise 'vitrified tiles' for exactly this reason. In nuclear work it is the standard method of dealing with dangerous liquid waste: mix it into molten glass, let it set, and the radioactive material is locked inside a solid block that will not leak. It also shows up in medicine and biology, where vitrification is the fast-freezing technique used on eggs and embryos: cool them so quickly that the water inside becomes a glass-like solid instead of forming ice crystals, which would cut the cell apart. And in geology, lightning or intense heat can vitrify sand.

So the common thread is not 'becoming a window'. It is 'becoming a solid with no orderly crystal structure inside' — smooth, sealed, and hard. This is a technical word only; in ordinary talk you would say 'turned to glass'.