a supercooled liquid
A supercooled liquid is a liquid that has been cooled below the temperature where it should have frozen, yet somehow has not. Very clean water in a smooth container can be chilled several degrees below 0 degrees C and stay stubbornly liquid, because ice needs a seed to start growing and there was nothing to nucleate on. A supercooled liquid is living on borrowed time: it is in the wrong state for its temperature, and the smallest disturbance can trigger it to crystallize all at once.
This state is the crucial middle ground of glassmaking. As a melt cools past its melting point Tm without crystallizing, it becomes a supercooled liquid. Keep cooling and its viscosity climbs relentlessly. Somewhere below Tm it passes through the danger zone where crystals nucleate and grow fastest, and if it survives that gauntlet its atoms eventually stiffen to a halt at Tg, where the supercooled liquid finally freezes into a glass. So a glass is nothing more than a supercooled liquid that got away: the same material, arrested before it could find its crystal.
The word to remember is metastable. A supercooled liquid is not at true equilibrium; the crystal is always the lower-energy state it would rather be in. It survives only because the path to the crystal is kinetically blocked, either by high viscosity or by the lack of a nucleus. Give it time, warmth, or a speck of dust, and it will crystallize. Understanding the supercooled liquid explains both why glasses can be made at all and why they can slowly devitrify if reheated into the wrong temperature window.
A hand-warmer packet is a bottle of supercooled sodium acetate solution. Click the little metal disc and you provide a nucleus; the whole liquid crystallizes in seconds, releasing heat and turning solid. Glass is the same trick played in reverse: cool a melt carefully so no nucleus ever gets a foothold, and the supercooled liquid freezes into a glass instead of a crystal.
A click-to-heat pad crystallizes a supercooled liquid on demand; glassmaking instead keeps the supercooled liquid from ever crystallizing.
Supercooled does not mean stable. The liquid is metastable and always poised to crystallize; a glass is just a supercooled liquid whose atoms have become too sluggish, below Tg, to complete the move.