An emulsion is a mixture in which tiny droplets of one liquid are dispersed through another liquid that does not normally mix with it, such as oil and water. Emulsions may be temporary or stabilized; they are not all permanently stable.
Vinaigrette is an example that often separates on standing. Mayonnaise is an oil-in-water emulsion stabilized by ingredients in egg yolk. Butter is mainly a water-in-fat emulsion. Whisking or blending forms droplets, while emulsifiers and other stabilizing ingredients help keep them dispersed.
Emulsion names the mixture; emulsifying names the process used to make it.