a glass
Watch honey being cooled. Warm, it flows easily; cold, it becomes thick and sluggish; put it in the freezer and it turns into a stiff, transparent solid you could crack with a hammer — yet if you looked at the atoms, they are still jumbled exactly as they were when it flowed, just no longer moving. That is the essence of a glass: a liquid that has been cooled so fast, and become so stiff, that its atoms are trapped mid-shuffle before they can arrange into a crystal. A glass is, quite literally, a frozen liquid — a solid with the disordered atomic arrangement of a liquid but the rigidity of a solid.
More precisely, a glass is an amorphous solid formed by cooling a liquid THROUGH the glass transition without letting it crystallise. As a liquid cools, its viscosity (its resistance to flow) climbs steeply; near the glass transition temperature Tg the viscosity reaches roughly 10^12 pascal-seconds — about a million-billion times thicker than water — and the atoms can no longer rearrange within any reasonable time. At that point the material stops behaving like a flowing liquid and becomes a rigid glass, but it keeps the liquid's tangled, non-periodic structure: sharp short-range order (well-defined nearest neighbours), no long-range order. This is what distinguishes a glass from other amorphous solids — a glass is specifically the product of quenching a melt through Tg, not, say, of vapour deposition or radiation damage.
The classic example is silica glass and the ordinary soda-lime glass of windows and bottles, whose atomic backbone is a continuous random network of silicon and oxygen. But the idea is far broader: sugar pulled into hard candy is a glass, many polymers are glassy below their Tg, and even metals can be made into glasses if cooled fast enough. One important honesty check: the old story that old church windows are thicker at the bottom because glass slowly flows over centuries is a MYTH. At room temperature silica glass is so enormously viscous that it would take far longer than the age of the universe to flow visibly; the thickness variation comes from how the panes were made, not from flow.
Ordinary window glass is soda-lime glass: roughly 70 percent silica (SiO2), plus soda (Na2O) and lime (CaO) added to lower the melting point so it can be worked. Its atoms form a continuous but irregular network of silicon-oxygen tetrahedra — the same tetrahedra found in quartz crystal, but linked at random angles so the pattern never repeats. Cool the melt slowly and it would crystallise into quartz; cool it at ordinary rates and it freezes as a glass.
A glass is a melt cooled fast enough to freeze before it can crystallise — a frozen liquid.
A glass is not a state of matter of its own and not a slow liquid at room temperature. It is a non-equilibrium amorphous solid, kinetically trapped: given truly infinite time it would eventually crystallise, but for silica glass that time vastly exceeds the age of the universe.