Emulsion
A smooth mixture of two liquids that do not normally mix, such as oil and water.
Also: emulsify
Oil and water separate when left alone. In an emulsion, one is broken into tiny droplets that stay spread through the other. This takes vigorous whisking or blending, and usually an emulsifier, a substance that holds the droplets in place. Egg yolk and mustard are common emulsifiers. Mayonnaise, hollandaise, vinaigrette, milk, cream and butter are all emulsions. If an emulsion is heated too much or mixed too fast, it can split back into its parts.