A colloid is a mixture of substances in which the particles of one substance are of greater than molecular size but are stabilized, at least for a time, with respect to forming larger particles or settling under gravity. The particles are referred to as the disperse phase while the other phase is termed the dispersion medium or continuous phase. Smoke is a colloidal suspension of solid particles in air. Fog is a colloidal suspension of water droplets. Milk is a colloidal suspension of oil droplets in water. Chemists are most often concerned with colloids in which solid or liquid particles are suspended in a liquid. The special properties of colloids result from the large contact area between particle and solvent, which may reach hundreds of square meters per gram.

An ingestible colloidal suspension of gold particles in oil was used as "potable gold" by medical alchemists. The first...