Zest - The rich outermost part of the rind of an orange, lemon, or other citrus fruit, which can be used as flavoring. On a lemon, zest is the yellow part of the peel (skin) on the outside of a lemon. The zest is shiny, brightly colored, and textured; it is the outer surface of the fruit which consumers can directly see.

The peel of a citrus fruit contains two (2) top layers:

The zest (the outermost part of the rind)

The pith (a white, fibrous membrane directly below the zest which helps to protect the fruit inside).

Lemon zest has an intense lemon/citrus flavor with very little bitterness. The bitterness is primarily found in the white part of the lemon (the white pith). The white pith will give your dish a bitter under taste. Carefully zest your citrus fruits leaving the pith on the fruit.

Before zesting, scrub the fruit with a sponge and warm, soapy water. Rinse it well and dry it with a paper towel. 

Lemon Zester (top of photo) - Traditional zester, which makes long, thin strands of zest, or a knife. Be sure to finely chop the strands with a knife. A tool for removing the zest of a lemon. A lemon zester allows one to remove the zest (and only the zest) in long strips. A zester has tiny cutting holes that create threadlike strips of peel.

Lemon Grater (such as the Microplane version - bottom of photo) - You will also note that many recipes call for grated zest or peel. In this case, use a fine grater to remove the peel. To zest with the Microplane, just rub the lemon in one direction against the little blades. Turn the lemon as you go so you remove only the yellow part.