Emulsion

Fluid system in which liquid droplets are dispersed in a liquid.

Note 1: The definition is based on the definition in ref.

Note 2: The droplets may be amorphous, liquid-crystalline, or any
mixture thereof.

Note 3: The diameters of the droplets constituting the dispersed phase
usually range from approximately 10 nm to 100 μm; i.e., the droplets
may exceed the usual size limits for colloidal particles.

Note 4: An emulsion is termed an oil/water (o/w) emulsion if the
dispersed phase is an organic material and the continuous phase is
water or an aqueous solution and is termed water/oil (w/o) if the dispersed
phase is water or an aqueous solution and the continuous phase is an
organic liquid (an "oil").

Note 5: A w/o emulsion is sometimes called an inverse emulsion.
The term "inverse emulsion" is misleading, suggesting incorrectly that
the emulsion has properties that are the opposite of those of an emulsion.
Its use is, therefore, not recommended.

An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions include vinaigrettes, homogenized milk, mayonnaise, and some cutting fluids for metal working.

The word "emulsion" comes from the Latin word for "to milk", as milk is an emulsion of fat and water, along with other components.

Two liquids can form different types of emulsions. As an example, oil and water can form, first, an oil-in-water emulsion, wherein the oil is the dispersed phase, and water is the dispersion medium. (Lipoproteins, as implemented by all complex living organisms, is one example of this.) Second, they can form a water-in-oil emulsion, wherein water is the dispersed phase and oil is the external phase. Multiple emulsions are also possible, including a "water-in-oil-in-water" emulsion and an "oil-in-water-in-oil" emulsion.

...
Wikipedia