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The Hazelnut or Cobnut.
A hazelnut, for that is what a filbert is.
There are 14-18 species of hazelnuts. The species Corylus maxima is from southeastern Europe and southwestern Asia. It is known as a "Filbert" in the U.S. Cobnuts are also a type of hazelnut grown in the U.K. and sometimes called Kentish Cobnuts.
filbert
A hazelnut