What is the emulsifying agent in a mayonnaise?

An emulsifier is an ingredient that causes to other ingredients to mix together when they wouldn't normally. A common example is mustard when used in a vinegarette. It causes the oil and vinegar to emulsify which means they stay combined and don't separate after mixing. Emulsifiers aren't perfect and sometimes the mixture separates after sitting for some time. But it definitely postpones the separation. Egg yolks are a common emulsification agent in foods like mayonnaise.