Colloidal dispersions are a class of soft materials. The term typically refers to a material consisting of solid particles spread throughout a fluid. 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 subgroup of colloidal dispersions. One can also find definitions of colloidal dispersions which include other combinations of solid, liquid, and gas phases.
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 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).