A solution is a homogeneous mixture: its composition is uniform at the scale at which the material is treated as a continuous sample.

A solution contains two or more components. The component that acts as the main mixing medium is called the solvent; the other components are solutes. In many ordinary solutions the solvent is the component present in the greatest amount, although the more important idea is that it determines the medium in which the other components are dispersed.

When water is the solvent, the mixture is an aqueous solution.

Solutions are not limited to liquids. Air is a gaseous solution of several gases, and some metal alloys are solid solutions. What defines a solution is homogeneous mixing, not a particular physical state.

A solution differs from a pure chemical substance. A pure substance has a fixed chemical identity and composition, whereas salt water can contain many different proportions of salt and water while remaining one homogeneous phase.

It also differs from a heterogeneous mixture. If excess solid salt remains visibly separate from salt water, the overall sample contains more than one phase even though the liquid portion itself is a homogeneous solution.

Terms such as dilute and concentrated compare the relative amount of solute, but they are qualitative. Quantitative concentration measures specify that composition precisely.