What does "filbert" mean?

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