The hazelnut or cobnut is the fruit of the hazel tree, Corylus avellana L. (Family Betulaceae) and is a true nut. It is roughly spherical, about 10 to 15 mm in diameter, with an outer fibrous husk surrounding a smooth shell. The husk falls off during ripening. The kernel of the seed is edible and used raw or roasted, or ground into a paste. The seed has a thin, dark brown skin which has a bitter flavour and is sometimes removed before cooking.

Hazelnuts are produced in southern Europe and in the American states of Oregon and Washington.

Hazelnut oil, pressed from hazelnuts, is strongly flavoured and used as a cooking oil.

Hazelnuts are extensively used in confectionery to make praline and also used in combination with chocolate for chocolate truffles and products such as Nutella.

The nut known as the filbert is closely related, but is the nut of the species Corylus maxima.