Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

walnut
     n 1: nut of any of various walnut trees having a wrinkled
          two-lobed seed with a hard shell
     2: hard dark-brown wood of any of various walnut trees; used
        especially for furniture and paneling
     3: any of various trees of the genus Juglans [syn: {walnut tree}]