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]