An emulsion is a mixture of two immiscible substances. One substance (the dispersed phase) is dispersed in the other (the continuous phase).

Examples of emulsions include butter, certain types of asphalt concrete, mayonnaise and cutting fluid for metalworking. In butter, a continuous phase of milk fat surrounds droplets of water.

20ml ampule of 1% propofol emulsion suitable for intravenous injection. The manufacturers emulsify the lipid soluble propofol in a mixture water, soy oil and egg lecithin.

Colloidal emulsions are stable, meaning that the one phase will remain dispersed in the other over time. Non-colloidal emulsions are unstable and will tend to separate into separate, non-emulsified phases, over time. For example, homemade oil and vinegar salad dressing is an unstable emulsion that will quickly separate unless shaken continuously. An emulsifier (also known as a surfactant) is a substance which stabilizes an emulsion. Lecithin (found in egg yolk) is a common food emulsifier. Both mayonnaise and Hollandaise sauce are emusions stabilized with egg yolk lecithin. Another type of emulsifier is detergent, which will bind to both oil and water, thus holding microscopic oil droplets in suspension. This principle is exploited in soap to remove grease from plates, etc.

Emulsions tend to have a cloudy appearance, because the many phase interfaces scatter light that passes through the emulsion.

An emulsion paint (often abbreviated to emulsion) is a water-based paint commonly used for painting indoor surfaces. Emulsion paints are also known as latex paints. It is so called because the polymer is formed through an emulsion polymerization whereby the monomers were emsulified in a water continuous phase. The polymer itself is not soluble in water and hence the paint is water resistant after it has dried. Residual surfactants in the paint as well as hydrolytic effects with some polymers cause the paint to still be susceptible to softening and, over time, degradation by water.