"Zest: an enjoyably exciting quality."
Like the word means outside of culinary circles, zest itself is a pretty exciting thing. Simply put, it is the peel of a citrus fruit. More specifically, however, the zest is obtained by scraping or cutting off the exocarp of the fruits instead of the whole peel. As you may have noticed having eaten oranges or lemons, there is a white part of the peel that is bitter. This is called the pith, and it is not desired when zesting a fruit because it taints the natural sweetness of the outer layer of peel. Typically people use zesters to obtain the pieces but if those are unavailable, paring knives or vegetable peelers are adequate substitutes.
Zest is full of aromatic natural oils which are useful for flavoring various desserts or drinks. These oils can also be infused in chocolates and other sweets. In cocktails, the zest is cut large and twisted to add to the lips of glasses. For the most part, zest in recipes refers to grated zest, and that is the most common form of zest used in cooking. You can find grated zest in savory dishes as well as sweet (like cheesecake!). It adds both a nice citrusy taste and an interesting texture.
This post is part of an A-Z series I am running for my blog category "Origins and Information" while I am in Vietnam with my family for July. Many of the posts in the series answer questions that were posed by friends/readers. If y'all enjoy the series, I will gladly run another in the future!