Colloidal dispersions are a class of soft materials. The term typically refers to a material consisting of solid particles spread throughout a liquid. According to Jones [1, p. 49], colloidal particles must have dimensions on the order of 10<math>\mu</math>m or smaller.
Witten [2, p. 113] distinguishes between colloids (solid particles in liquid), emulsions (liquid drops in another liquid), and foams (gas bubbles in a liquid), while Jones [1, p. 1] includes emulsions as a subcategory of colloidal dispersions. One can also find definitions of colloidal dispersions which include other combinations of solid, liquid, and gas phases.
The particles in a colloidal dispersion are collectively called the dispersed phase, and the material between the particles is the continuous phase.
 R. Jones, "Soft Condensed Matter," Oxford University Press Inc., New York (2002).
 T. Witten, "Structured Fluids: Polymers, Colloids, Surfactants," Oxford University Press Inc., New York (2004).