A walnut is the nut of any tree of the genus Juglans , particularly the Persian or English walnut, Walnuts are rounded, single-seeded stone fruits of the walnut tree commonly used for the meat after fully ripening. Following full ripening, the removal of the husk reveals the wrinkly walnut shell, which is usually commercially found in two segments (three-segment shells can also form). During the ripening process, the husk will become brittle and the shell hard. The shell encloses the kernel or meat, which is usually made up of two halves separated by a partition.

Walnuts are late to grow leaves, typically not until more than halfway through the spring. They secrete chemicals into the soil to prevent competing vegetation from growing. Because of this, flowers or vegetable gardens should not be planted close to them.

The two most common major species of walnuts are grown for their seeds – the Persian or English walnut and the black walnut.

Walnut meats are available in two forms; in their shells or shelled. The meats can be as large as halves or any smaller portions that may happen during processing, candied or as an ingredient in other foodstuffs. Pickled walnuts that are the whole fruit can be savory or sweet depending on the preserving solution. Walnut butters can be homemade or purchased in both raw and roasted forms.