What is "Zest" - In particular: lime/lemon zest?
The very thin outside layer of the Lime, Lemon, Orange, and other citrus. It contains aromatic and flavorful oils that will enhance your meals.

When you zest your citrus fruit (usually with a fine grater) you need make sure that you only pull off the zest and not the white pith that lies underneath. The pith is bitter and generally not something you want in your dish.