Also called frosting, this sweet sugar-based mixture is used to fill and coat cakes, pastries, cookies, etc. In addition to sugar, icing can contain a combination of other ingredients including butter, milk, water, eggs and various flavorings. It can be cooked (as with boiled icing) or uncooked (as with buttercream) and can range from thick to thin. Icing must be thick enough to adhere to the item being coated, yet soft enough to spread easily.
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.