A solution is a mixture in which two or more substances are evenly distributed at the particle level. It is a homogeneous mixture, meaning that a sample taken from one part has the same composition as a sample taken from another part.
The substance present in the greater amount is usually called the solvent, while the substance dissolved into it is the solute. In saltwater, for example, water is the solvent and salt is the solute. The salt does not disappear; its particles spread throughout the water and become too small to see individually.
Solutions can involve states of matter other than solid dissolved in liquid. Air is a gaseous solution of several gases, and brass is a solid solution of copper and zinc. Solutions may also be colored, such as copper sulfate dissolved in water, but color is not required.
A solution differs from a suspension, such as muddy water, because suspended particles are large enough to settle out. It also differs from a colloid, such as milk, whose particles remain dispersed but are larger than the particles in a true solution.