Emulsions are systems composed of two or more immiscible materials, in which one material (the discontinuous or internal phase) is suspended or dispersed throughout another material (the continuous or external phase) in separate droplets. The immiscible phases can be water, oil or silicone. When emulsions are made surfactants called emulsifiers are used to slow the process of separation of the immiscible phases. All emulsions are inherently unstable with the exception of some spontaneously forming micro emulsions. Simply put, any combination of unlike phases that are put together can be considered a type of emulsion.
Emulsions are classified by the continuous phase (external) and the discontinuous phase (internal). The use of homogenizers and other equipment to minimize droplet size will improve the stability of an emulsion. When naming the emulsion type, the first letter is the discontinuous phase. O/W stands for oil-in-water and is classified as an emulsion. W/O stands for water-in-oil and is classified as an invert emulsion.