Mixtures can be either homogeneous or heterogeneous: a mixture of uniform composition and in which all components are in the same phase, such as salt in water, is called homogeneous, whereas a mixture of non-uniform composition and of which the components can be easily identified, such as sand in water, it is called heterogeneous.
In addition, "uniform mixture" is another term for homogeneous mixture and "non-uniform mixture" is another term for heterogeneous mixture. These terms are derived from the idea that a homogeneous mixture has a uniform appearance, or only one visible phase, because the particles are evenly distributed. However, a heterogeneous mixture has non-uniform composition, and its constituent substances are easily distinguishable from one another (often, but not always, in different phases).
Several solid substances, such as salt and sugar, dissolve in water to form a special type of homogeneous mixture called a solution, in which there is both a solute (dissolved substance) and solvent (dissolving medium) present. Air is an example of a solution as well: a homogeneous mixture of gaseous nitrogen solvent, in which oxygen and smaller amounts of other gaseous solutes are dissolved. Mixtures are not limited in either their number of substances or the amounts of those substances, though in a homogeneous mixture the solute-to-solvent proportion can only reach a certain point before the mixture separates and becomes heterogeneous.
A homogeneous mixture is characterized by uniform dispersion of its constituent substances throughout; the substances exist in equal proportion everywhere within the mixture. Differently put, a homogeneous mixture will be the same no matter from where in the mixture it is sampled. For example, if a solid-liquid solution is divided into two halves of equal volume, the halves will contain equal amounts of both the liquid medium and dissolved solid (solvent and solute).
In physical chemistry and materials science, "homogeneous" more narrowly describes substances and mixtures which are in a single phase.