An emulsion is a stabilized mixture which contains two or more liquids that wouldn’t ordinarily combine, such as oil and vinegar. Some emulsions are temporary such as a homemade vinaigrette however, other emulsions like mayonnaise are more stable and permanent.
All emulsions require vigorous blending in order to get the ingredients to combine. A stable emulsion (mayo) will require a emulsifier to help hold the ingredients together. A common and popular emulsifier is egg yolks. Mustard and cream can also be used.