An emulsion in cosmetics is best described as:

An emulsion is a mixture of two liquids that normally do not mix — such as oil and water — combined and held together with the help of an emulsifier (a substance that keeps the droplets dispersed). Many cosmetic products, like lotions and creams, are emulsions. The emulsifier surrounds the droplets of one liquid so they stay suspended in the other rather than separating. Understanding emulsions and basic product chemistry helps cosmetologists understand how the products they use are formulated and behave.