A walnut is an edible seed of any tree of the genus Juglans, especially the Persian or English walnut, Juglans regia. Broken nutmeats of the eastern black walnut from the tree Juglans nigra are also commercially available in small quantities, as are foods prepared with butternut nutmeats from Juglans cinerea. Walnut seeds are a high density source of nutrients, particularly proteins and essential fatty acids. Walnuts, like other tree nuts, must be processed and stored properly. Poor storage makes walnuts susceptible to insect and fungal mold infestations; the latter produces aflatoxin - a potent carcinogen. A mold infested walnut seed batch should not be screened and then consumed; the entire batch should be discarded. Walnuts are rounded, single-seeded stone fruits of the walnut tree. The walnut fruit is enclosed in a green, leathery, fleshy husk. This husk is inedible. After harvest, the removal of the husk reveals the wrinkly walnut shell, which is in two halves.
Contributions by ApostleVonColorado, Jay L09, and ClueBot NG.