A colloid or colloidal dispersion, is a form of matter intermediate between a true solution and a mixture (suspension). Microscopic particles of one substance, said to be in the dispersed or solute phase, are distributed throughout another, said to be in the dispersing, continuous, or solvent phase.

The field was introduced in 1861 by Scottish scientist Thomas Graham.

Colloidal sizes range from 0.001 to 1 micrometers in any dimension. Dispersions where the particle size is in this range are referred to as colloidal aerosols, colloidal emulsions, colloidal foams, or colloidal suspensions or dispersions. Colloids may be colored or translucent because of the Tyndall effect. The Tyndall effect is the scattering of light by particles in the colloid.

Colloids can be classified as follows:

		Dispersed Phase
		Gas	Liquid	Solid
Dispersing Phase	Gas	None>	Liquid aerosol, Examples: fog, mist	Solid aerosol, Examples: Smoke, dust
	Liquid	Foam, Examples: Whipped cream	Emulsion, Examples: Milk, mayonnaise, hand cream, blood	Sol, Examples: Paint, ink
	Solid	Solid foam , Examples: Styrofoam	Gel, Examples: Gelatin, jelly, cheese	Solid sol , Examples: Ruby glass

In the early 1900s, before enzymology was well understood, colloids were thought to be the key to the operation of enzymes; i.e., the addition of small quantities of an enzyme to a quantity of water would, in some fashion yet to be specified, subtly alter the properties of the water so that it would break down the enzyme's specific substrate, such as a solution of ATPase breaking down ATP. Furthermore, life itself was explainable in terms of the aggregate properties of all the colloidal substances that make up an organism. As more detailed knowledge of biology and biochemistry developed, of course, the colloidal theory was replaced by the macromolecular theory, which explains an enzyme as a collection of identical huge molecules which act as very tiny machines, freely moving about between the water molecules of the solution and individually operating on the substrate, no more mysterious than a factory full of machinery. The properties of the water in the solution are not altered, other than the simple osmotic changes that would be caused by the presence of any solute.