Real Dictionary

walnut
countable noun: Walnuts are edible nuts which have a wrinkled shape and a hard round shell that is light brown in colour.

walnut in American English
any of a genus (Juglans) of trees of the walnut family, valued as shade trees, for their nuts, and for their wood, used in making furniture, paneling, etc.
designating a family (Juglandaceae, order Juglandales) of dicotyledonous trees native to the temperate parts of the Northern Hemisphere, including the black walnut, English walnut, pecan, and hickory
noun: the edible nut of trees of the genus Juglans, of the North Temperate Zone

walnut in British English
adjective: made from the wood of a walnut tree
noun: any juglandaceous deciduous tree of the genus Juglans, of America, SE Europe, and Asia, esp J. regia, which is native to W Asia but introduced elsewhere. They have aromatic leaves and flowers in catkins and are grown for their edible nuts and for their wo