A walnut is a seed from a tree in the genus Juglans. Technically, a walnut is a drupe, not a nut, since it takes the form of a fruit enclosed by a fleshy outer layer which parts to reveal a thin shell with a seed inside. As walnuts age on the tree, the outer shell dries and pulls away, leaving the shell and seed behind. Whether you call it a nut or a drupe, walnuts can pose risks to people with allergies, so use them with caution in cooking. It is a good idea to get in the habit of disclosing all of the ingredients in a dish to cope with allergy concerns and dietary restrictions.

The Juglans genus is extremely large and well distributed. The trees have simple, pinnately compound leaves with resinous spots. The odor of the resin is quite distinctive, and the resin can be harmful to plants grown underneath walnut trees, which is why the ground under them tends to be bare. Representative trees can be found all over the world, although they are primarily concentrated in the Northern hemisphere. Walnuts are also found growing in Africa and the Southern reaches of the Americas. The nuts have been used in both sweet and savory dishes for centuries, with some species being more favored than others.

One of the most popular varieties of walnut is the Persian or English walnut, which has a large seed and a thinner shell, yielding more edible meat per pound than other species. The Persian walnut came from the Middle East originally, although England established a monopoly on the trade at one point, leading to the associations with both regions of the Earth. Black walnuts are another commonly sold walnut species, as are white walnuts, also called butternuts. Some species such as black walnut are cultivated for their wood as well as the edible drupes.

Commercially, walnuts are available both whole and shelled. Many consumers prefer to purchase shelled walnuts, since they can look for plump, glossy walnuts. Shriveled walnuts are not good to eat, and they can lurk inside any type of shell, which can be frustrating. Like many nuts, walnuts can be frozen until use, or stored in a cool, dry, dark place for several months. They are also pressed to yield their dark, rich, flavorful oil.

Walnuts often appear paired with blue cheese in salads, mixed into stuffings, or in Middle Eastern cuisine with ingredients like raisins. They can also be candied for use in desserts, or used plain to provide a bitter contrast to sweet fruits. The uses for the walnut are myriad, which explains its enduring popularity as a cash crop.