In general, a colloid or colloidal dispersion, is a two-phase system of matter; small droplets or particles of one substance, the dispersed phase, are dispersed in another, continuous phase forming so called phase colloid. Another type of colloid is called molecular colloid and is formed of macromolecules dispersed in a continuous phase (dispersion medium). Many familar substances, including butter, milk, cream, aerosols (fog, smog, smoke), asphalt, inks, paints, glues and sea foam, are colloids. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

The size of dispersed phase particles in a colloid range from 0.001 to 1 micrometers. 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:

Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. Different phases generally have different charge affinities, so that a charge double-layer forms at any interface. Small particle sizes lead to enormous surface areas, so that this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too little to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particles can be observed by applying an electric field: all particles migrate to the same electrode and therefore must all have the same charge.

The destruction of a colloid, called coagulation, can be accomplished by heating or by adding an electrolyte. Heating increases the velocities of the particles, causing them to collide with enough energy that the barriers are penetrated and the particles can aggregate. Since this is repeated many times, the particle grows to be large enough to form a precipitate. Adding an electrolyte neutralizes the absorbed ion layers.

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.