A substance used to aid in keeping two non-mixable substances even dispersed in small particles. An example would be oil and water. Without an emulsifier these two substances would immediately separate and form two layers. With an emulsifier, they will stay mixed together. A common emulsifier is egg yolk (more specifically, the lecithin in the egg yolk). Egg yolk is used with oil and water to make mayonnaise, a stable emulsification.