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The Collaborative International Dictionary
Corylus maxima

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.

  2. (Bot.) The tree bearing the filbert; the hazelnut tree.

    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).

Wikipedia
Corylus maxima

Corylus maxima, the filbert, is a species of hazel native to southeastern Europe and southwestern Asia, from the Balkans to Ordu in Turkey.

It is a deciduous shrub tall, with stems up to thick. The leaves are rounded, 5–12 cm long by 4–10 cm broad, with a coarsely double-serrated margin. The flowers are wind-pollinated catkins produced in late winter; the male ( pollen) catkins are pale yellow, 5–10 cm long, while the female catkins are bright red and only 1–3 mm long. The fruit is a nut produced in clusters of 1–5 together; each nut is 1.5–2.5 cm long, fully enclosed in a 3–5 cm long, tubular involucre ( husk).

The filbert is similar to the related common hazel, C. avellana, differing in having the nut more fully enclosed by the tubular involucre. This feature is shared by the beaked hazel C. cornuta of North America, and the Asian beaked hazel C. sieboldiana of eastern Asia.