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.
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.
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).