A walnut is the edible seed of any tree of the genus Juglans, particularly the Persian or English walnut, Juglans regia. The fruits of trees in the family Juglandaceae are often confused with drupes.
Walnuts are the round, single-seed stone fruits of the walnut tree. They are commonly used for food. They ripen between September and November in the northern hemisphere. The brown, wrinkly walnut shell is enclosed in a husk. Shells of walnuts available in commerce usually have two segments (but three or four-segment shells can also form). During the ripening process, the husk becomes brittle and the shell hard. The shell encloses the kernel or meat, which is usually in two halves separated by a membranous partition. The seed kernels – commonly available as shelled walnuts – are enclosed in a brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen, preventing rancidity.