filbert, 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.].

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]
" 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]
The tree bearing the filbert; the hazelnut tree. [PJC]

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

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OXFORD DICTIONARY

filbert, n.
1 the cultivated hazel, Corylus maxima, bearing edible ovoid nuts.
2 this nut.

Etymology

ME philliberd etc. f. AF philbert, dial. F noix de filbert, a nut ripe about St Philibert's day (20 Aug.)

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