Walnut \Wal"nut\, n. [OE. walnot, AS. wealh-hnutu a Welsh or foreign nut, a walnut; wealh foreign, strange, n., a Welshman, Celt (akin to OHG. Walh, properly, a Celt, from the name of a Celtic tribe, in L. Volcae) + hnutu a nut; akin to D. walnoot, G. walnuss, Icel. valhnot, Sw. valn["o]t, Dan valn["o]d. See Nut, and cf. Welsh.]

(Bot.) The fruit or nut of any tree of the genus Juglans; also, the tree, and its timber. The seven or eight known species are all natives of the north temperate zone.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Note: In some parts of America, especially in New England, the name walnut is given to several species of hickory (Carya), and their fruit.

Ash-leaved walnut, a tree (Juglans fraxinifolia), native in Transcaucasia.

Black walnut, a North American tree (J. nigra) valuable for its purplish brown wood, which is extensively used in cabinetwork and for gunstocks. The nuts are thick-shelled, and nearly globular.

English, or European, walnut, a tree (J. regia), native of Asia from the Caucasus to Japan, valuable for its timber and for its excellent nuts, which are also called Madeira nuts.

Walnut brown, a deep warm brown color, like that of the heartwood of the black walnut.

Walnut oil, oil extracted from walnut meats. It is used in cooking, making soap, etc.

White walnut, a North American tree (J. cinerea), bearing long, oval, thick-shelled, oily nuts, commonly called butternuts. See Butternut.

Source: Webster's Revised Unabridged Dictionary (1913)