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["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.
   [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]

WordNet (r) 3.0 (2006):

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,
       walnut tree]

U.S. Gazetteer Places (2000):

Walnut, CA -- U.S. city in California
   Population (2000):    30004
   Housing Units (2000): 8395
   Land area (2000):     8.982015 sq. miles (23.263311 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    8.982015 sq. miles (23.263311 sq. km)
   FIPS code:            83332
   Located within:       California (CA), FIPS 06
   Location:             34.027738 N, 117.860514 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Walnut, CA
    Walnut


U.S. Gazetteer Places (2000):

Walnut, IL -- U.S. village in Illinois
   Population (2000):    1461
   Housing Units (2000): 607
   Land area (2000):     0.832855 sq. miles (2.157084 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.832855 sq. miles (2.157084 sq. km)
   FIPS code:            78526
   Located within:       Illinois (IL), FIPS 17
   Location:             41.556404 N, 89.591503 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Walnut, IL
    Walnut


U.S. Gazetteer Places (2000):

Walnut, IA -- U.S. city in Iowa
   Population (2000):    778
   Housing Units (2000): 350
   Land area (2000):     2.144188 sq. miles (5.553422 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.144188 sq. miles (5.553422 sq. km)
   FIPS code:            82065
   Located within:       Iowa (IA), FIPS 19
   Location:             41.480999 N, 95.221116 W
   ZIP Codes (1990):     51577
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Walnut, IA
    Walnut


U.S. Gazetteer Places (2000):

Walnut, KS -- U.S. city in Kansas
   Population (2000):    221
   Housing Units (2000): 116
   Land area (2000):     0.990805 sq. miles (2.566174 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.990805 sq. miles (2.566174 sq. km)
   FIPS code:            74950
   Located within:       Kansas (KS), FIPS 20
   Location:             37.600359 N, 95.075867 W
   ZIP Codes (1990):     66780
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Walnut, KS
    Walnut


U.S. Gazetteer Places (2000):

Walnut, MS -- U.S. town in Mississippi
   Population (2000):    754
   Housing Units (2000): 341
   Land area (2000):     5.420013 sq. miles (14.037768 sq. km)
   Water area (2000):    0.024984 sq. miles (0.064708 sq. km)
   Total area (2000):    5.444997 sq. miles (14.102476 sq. km)
   FIPS code:            77480
   Located within:       Mississippi (MS), FIPS 28
   Location:             34.949109 N, 88.909812 W
   ZIP Codes (1990):     38683
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Walnut, MS
    Walnut