Emulsion
A colloidal dispersion of one liquid as fine droplets throughout another liquid with which it is immiscible.
Unit
nm
In depth
Emulsions are inherently unstable and separate unless an emulsifying agent is present to lower the interfacial tension and coat the droplets. They are classified as oil-in-water or water-in-oil, and the type is determined by which phase the emulsifier is more soluble in.
Examples in the real world
Milk (fat in water, stabilised by casein), mayonnaise (oil in water, stabilised by egg lecithin), and butter.