Corylus avellana, the common hazel, is a species of flowering plant in the birch family Betulaceae.

The name 'hazelnut' applies to the nuts of any species in the genus Corylus, but in commercial contexts usually describes C. avellana.

This hazelnut or cob nut, the kernel of the seed, is edible and used raw, roasted, or ground into a paste.

The leaves are deciduous, rounded, 6–12 centimetres (2+1⁄2–4+1⁄2 inches) long and across, softly hairy on both surfaces, and with a double-serrate margin.

[2][8][3] The leaves provide food for many animals, including Lepidoptera such as the case-bearer moth Coleophora anatipennella.

Caterpillars of the concealer moth Alabonia geoffrella have been found feeding inside dead common hazel twigs.

The fruit are possibly even more important animal food, both for invertebrates adapted to circumvent the shell (usually by ovipositing in the female flowers, which also gives protection to the offspring) and for vertebrates which manage to crack them open (such as squirrels and corvids).

The wood was traditionally grown as coppice, the poles cut and used for wattle-and-daub building and agricultural fencing.

[18] Common hazel is cultivated for its nuts in commercial orchards in Europe, Turkey, Iran and Caucasus.