zest [zest] noun, verb
The colored, outermost surface of citrus fruit skin. Recipes often suggest that the skin be grated or “zested” into tiny pieces. Underneath is the pith, which is whitish, sometimes cottony in texture and can be bitter. It lacks the flavorful natural oils and color of the zest.
Citrus zest is most easily removed by using a Microplane zester. The blades of this tool are so sharp that they easily grab the colored skin, leaving the white pith behind. The classic box grater is more aggressive and often cuts too deeply, bringing the pith along. No matter what tool you use, take care just to remove the outermost layer, unless the recipes recommends otherwise.