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 (p. 49), colloidal particles must have dimensions on the order of 10<math>\mu</math>m or smaller.
Witten (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 colloids. One can also find definitions for colloidal dispersions which include other combinations of solid, liquid, and gas phases.
 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).