The perfumy outermost skin layer of citrus fruit (usually oranges or lemons), which is removed with the aid of a citrus zester, paring knife or vegetable peeler. Only the colored portion of the skin (and not the white pith) is considered the zest. The aromatic oils in citrus zest are what add so much flavor to food. Zest can be used to flavor raw or cooked and sweet or savory dishes.
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.