Source : Webster's Revised Unabridged Dictionary (1913)

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.] (Bot.)
   The fruit of the {Corylus Avellana} or hazel. It is an oval
   nut, containing a kernel that has a mild, farinaceous, oily
   taste, agreeable to the palate.

   Note: In England filberts are usually large hazelnuts,
         especially the nuts from selected and cultivated trees.
         The American hazelnuts are of two other species.

   {Filbert gall} (Zo["o]l.), a gall resembling a filbert in
      form, growing in clusters on grapevines. It is produced by
      the larva of a gallfly ({Cecidomyia}).

Source : WordNet®

filbert
     n 1: small nut-bearing tree much grown in Europe [syn: {cobnut},
          {Corylus avellana}, {Corylus avellana grandis}]
     2: nut of any of several trees of the genus Corylus [syn: {hazelnut},
         {cobnut}, {cob}]