saturated solution

A saturated solution is a glass of water that simply cannot take any more sugar. You can keep stirring, but the extra solid just sits there undissolved — the liquid has hit its limit and pushes back as fast as new molecules try to dissolve.

Technically, a saturated solution is one in equilibrium with excess undissolved solid: molecules are still leaving the solid and entering solution, but at exactly the same rate they are leaving solution and rejoining the solid, so the dissolved concentration stays constant. That constant concentration is, by definition, the drug's solubility at that temperature. A solution holding less than this is unsaturated; one transiently holding more is supersaturated and unstable.

Because saturation depends on temperature, a solution saturated when warm can become supersaturated as it cools, risking crystals forming in the product. Measuring solubility correctly therefore means equilibrating excess solid with solvent at a fixed temperature for long enough that the concentration stops changing, then filtering and assaying the clear liquid.