[1] : The Collaborative International Dictionary of English v.0.48

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"ot, dan
   valn"od. 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.
   1913 webster
   1913 webster

   note: in some parts of america, especially in new england,
         the name walnut is given to several species of hickory
         carya, and their fruit.
         1913 webster

   ash-leaved walnut, a tree juglans fraxinifolia, native
      in transcaucasia.

   black walnut, a north american tree juglans 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 walnut, or european walnut, a tree juglans
      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 juglans cinerea,
      bearing long, oval, thick-shelled, oily nuts, commonly
      called butternuts. see butternut.
      1913 webster
see also:
nut welsh juglans carya ash-leaved walnut juglans fraxinifolia 
black walnut juglans nigra english walnut european walnut juglans regia 
madeira nuts walnut brown walnut oil white walnut juglans cinerea 
butternuts butternut 

[2] : WordNet (r) 2.0

juglans nigra
     n : north american walnut tree with hard dark wood and edible
         nut syn: black walnut, black walnut tree, black
         hickory
see also:
black walnut black walnut tree black hickory 

Nigra >> N Words
Nigra, definition of term: Nigra
nigra_pag1.html

Juglans >> J Words
Juglans, definition of term: Juglans
juglans_pag1.html