Solubility refers to the ability for a given substance, the solute, to dissolve in a solvent. It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution. Certain substances are soluble in all proportions with a given solvent. An example of this is ethanol in water. This property is more correctly described as miscible. Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable. The solvent is often a liquid, which can be a pure substance or a mixture. The species that dissolves, the solute, can be a gas, another liquid, or a solid. Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water. The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.

Solubility is commonly expressed as a concentration, either molarity or molality, but also as a mole fraction. The maximum equilibrium amount of solute that can normally dissolve per amount of solvent is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. These maximum concentrations are often expressed as grams of solute per 100 ml of solvent.

Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility (see solubility equilibrium). For salts, solubility in aqueous solutions or the maximum amount of salt that can be dissolved is the solubility constant. The solubility constant is a special case of an equilibrium constant. It describes the balance between dissolved salt and undissolved salt. The solubility constant is also "applicable" (i.e. useful) to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant.

Henry's law is used to quantify the solubility of gases in liquids as a function of the gas's partial pressure.

The Flory-Huggins solution theory is a theoretical model describing the solubility of polymers. The Hansen Solubility Parameters and the Hildebrand solubility parameters are empirical methods for the prediction of solubility. it is also possible to predict solubility from other physical constants such as the enthalpy of fusion.

The partition coefficient (Log P) is a measure of differential solubility of a compound in a hydrophobic solvent (octanol) and a hydrophilic solvent (water). The logarithm of these two values enables compounds to be ranked in terms of hydrophilicity (or hydrophobicity).