What does 'emulsion' mean?

An emulsion is the stable combination of two liquids that normally don't mix, like oil and water, achieved through vigorous mixing or adding an emulsifier like egg yolks or mustard. 

In baking, common emulsions include mayonnaise, hollandaise, buttercream, and vinaigrettes. Home bakers create emulsions to achieve smooth, creamy textures and prevent separation in sauces and frostings.

Emulsions can "break" or "split" when ingredients are too hot, too cold, or added too quickly, causing the mixture to curdle or separate. To fix a broken emulsion, gradually whisk the broken mixture into a fresh egg yolk or small amount of warm water. Prevention is key: ensure ingredients are at proper temperature and add oil or butter very slowly while whisking constantly.