Hazel, in botany, a genus of the monoecia polyandra clafs. The calix of the male consists of one trifid leaf, and contains but one flower; it has no corolla, but eight stamens: the calyx of the female consists of two lacerated leaves; it has no corolla; the stigmas are two; and the nut is oval. There are two species, viz. the avellana, a native of Britain; and the colurna, a native of Byzantium.