Depends on where you draw the line. Or whether there is a line (there isn’t).
If there are three distinct states of matter at STP, then solids are materials that have a crystalline structure, where atoms are bonded to one another in a regular, repeating pattern. Liquids are materials in which the atoms (or individual molecules) are not bonded to one another, and slide past one another when subjected to stress.
The truth however, is that there is a broad DMZ at the boundary between solids and liquids, and glass lives here. Each silicon atom is bonded to several oxygen atoms; there is no regular, repeating pattern to the structure (it is not crystalline), but the individual atoms are not free to slide about when the material is subjected to moderate stresses.
In this irregular structure, the bonds are weaker than those in crystalline SiO2 (quartz). This means, for instance, that glass is not as hard as quartz. It also means that the bonds can slip past one another under stress, but very, very slowly. Left on its own, after millions of years, glass will flow a bit under stress; but eventually the system of atoms will fall to their lowest energy state, and the system will begin to crystallize.
Sorry to trash your science education Greg, but it’s happened to us all. If you’d like the rest of the structure swept from beneath you, go to: http://www.ems.psu.edu/~fraser/BadScience.html