In the culinary arts, an emulsion is a mixture of two liquids that would ordinarily not mix together, like oil and vinegar.
There are two kinds of emulsions, temporary and permanent. An example of a temporary emulsion is a simple vinaigrette. You combine the oil and vinegar in a jar, mix them up and they come together for a short time. Mayonnaise is an example of a permanent emulsion, consisting of egg yolks and oil.
Egg yolks and oil would not naturally mix together, but by slowly whisking the oil into the egg yolks, the two liquids form a stable emulsion that won't separate.
Hollandaise sauce is another permanent emulsion, which is made of egg yolks and clarified butter. Clarified butter is best for forming the emulsion because whole butter contains around 15 percent water, and this water can destabilize the emulsion.
Certain substances act as emulsifiers, which means they help the two liquids come together and stay together. In the case of mayonnaise and Hollandaise, it is the lecithin in the egg yolks that acts as the emulsifier. Lecithin, a fatty substance that is soluble in both fat and water, will readily combine with both the egg yolk and the oil or butter, essentially holding the two liquids together.
In a stable emulsion, what happens is that droplets of one of the liquids become evenly dispersed within the other liquid.
The resulting liquid is thicker than the two original liquids were. In the case of salad dressing, oil droplets are suspended within the vinegar.
A cornstarch slurry works the same way.
One less obvious example is chocolate, which an emulsion of milk and cocoa butter. In fact, milk itself is an emulsion of water, protein solids and butterfat. If you've ever added lemon juice to milk, or maybe boiled it, you've seen what curdled milk looks like. Curdling is the breaking of the emulsion, which causes the protein solids to coagulate and separate from the liquid.
Another surprising example of something which is technically an emulsion is certain types of sausages and forcemeats. Hot dogs are an emulsified sausage where meat, fat and water are combined to form a smooth filling which is then stuffed into a casing.