Generally, any ingredient used to bind together normally noncombinative substances, such as oil and water. Egg yolks contain a natural emulsifier (lecithin) and are used to thicken and bind sauces (such as hollandaise), as well as to bind ingredients in baking. Xanthan gum is a commercial emulsifier used in numerous foods like salad dressings and dairy products. Some commercial emulsifiers also inhibit baked goods from going stale.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.