The ratio of observed solubility to that of equilibrium solubility is the degree of supersaturation

Supersaturation.

Dizzy X

Yes, under pressure.

Supersaturation is the driving force for crystallization. Supersaturation is where a liquid (solvent) contains more dissolved solid (solute) that can be ordinarily accomodated at that temperature.

A state of supersaturation can be achieved a combination of high solute concentration and rapid cooling.

Also seeding can initiate the initial nucleation process. (Where nucleation is the initial process by which a crystal grows.)