Juglans regia, the Persian walnut, English walnut, Carpathian walnut, Madeira walnut, or, especially in Great Britain, common walnut, is an Old World walnut tree species native to the region stretching from the Caucasus eastward to the Kashmir region. It is widely cultivated across Asia, Northern America and Europe.

The Juglandaceae are a plant family known as the walnut family. They are trees, or sometimes shrubs, in the order Fagales. Members of this family are native to the Americas, Eurasia, and Southeast Asia.