The Collaborative International Dictionary of English v.0.48:

Hazel \Ha"zel\ (h[=a]"z'l), n. [OE. hasel, AS. h[ae]sel; akin to
   D. hazelaar, G. hazel, OHG. hasal, hasala, Icel. hasl, Dan &
   Sw. hassel, L. corylus, for cosylus.]
   1. (Bot.) A shrub or small tree of the genus Corylus, as
      the Corylus avellana, bearing a nut containing a kernel
      of a mild, farinaceous taste; the filbert. The American
      species are Corylus Americana, which produces the common
      hazelnut, and Corylus rostrata. See Filbert. --Gray.
      [1913 Webster]

   2. A miner's name for freestone. --Raymond.
      [1913 Webster]

   Hazel earth, soil suitable for the hazel; a fertile loam.


   Hazel grouse (Zool.), a European grouse (Bonasa
      betulina), allied to the American ruffed grouse.

   Hazel hoe, a kind of grub hoe.

   Witch hazel. See Witch-hazel, and Hamamelis.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

filbert \fil"bert\, n. [Perh. fr. fill + bread, as filling the
   bread or husk; cf. G. bartnuss (lit., bread nut) filbert; or
   perh. named from a St.Philibert, whose day, Aug. 22, fell in
   the nutting season.]
   1. (Bot.) The fruit of the Corylus Avellana or Corylus
      maxima, also called the hazel; the hazelnut. It is an
      oval nut, containing a kernel that has a mild,
      farinaceous, oily taste, agreeable to the palate.
      [1913 Webster]

   Note: In England filberts are usually large hazelnuts,
         especially the nuts from selected and cultivated trees.
         The American hazelnuts are of two other species,
         Corylus Americana and Corylus cornuta, and are also
         sometimes called filberts.
         [1913 Webster +PJC]

   2. (Bot.) The tree bearing the filbert; the hazelnut tree.
      [PJC]

   Filbert gall (Zool.), a gall resembling a filbert in form,
      growing in clusters on grapevines. It is produced by the
      larva of a gallfly (Cecidomyia).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Corylus avellana
    n 1: small nut-bearing tree much grown in Europe [syn: cobnut,
         filbert, Corylus avellana, Corylus avellana grandis]