The outer colored part of peel on a citrus fruit. Such as "lemon zest" or "orange zest." It contains a lot of aromatic oils and flavor that help enhance many dishes.
You can harvest the zest by running a citrus zester (fine grater, vegetable peeler or paring knife) across the fruit. Be sure to only get the first layer of freshly washed fruit. If you can see the white part, you've zested too far.