Is glass a liquid or a solid?

There’s one thing unusual concerning the stained-glass home windows of medieval buildings: They are usually thicker on the backside than on the high. Many imagine it’s because glass is definitely a liquid that has flowed down the pane over the centuries. In distinction, others argue that tumbler doesn’t move, as a result of it is a strong — or possibly an amorphous strong or a supercooled liquid.

So what’s glass, actually — a strong or a liquid, or one thing in between?