Supersaturated Solution
Definition
A solution holding MORE dissolved solute than should be possible: an unstable state that crystallizes dramatically when disturbed.
Technical definition
A supersaturated solution contains more dissolved solute than the equilibrium solubility at that temperature, typically prepared by cooling a saturated hot solution carefully. It is metastable, and seeding or agitation triggers rapid crystallization.
Example
A hand warmer's sodium acetate solution stays liquid until you click the metal disc, then it crystallizes instantly, releasing heat.
Common confusion
Supersaturated solutions are not at equilibrium. They are temporarily stable accidents: the excess solute wants to precipitate and will at the slightest provocation.