The hazelnut is the fruit of the hazel and therefore includes any of the nuts deriving from species of the genus Corylus, especially the nuts of the species Corylus avellana. They are also known as cobnuts or filberts according to species. A cob is roughly spherical to oval, about 15–25 mm (5⁄8–1 in) long and 10–15 mm (3⁄8–5⁄8 in) in diameter, with an outer fibrous husk surrounding a smooth shell and a filbert is more elongated, being about twice as long as its diameter. The nut falls out of the husk when ripe, about seven to eight months after pollination. 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 is sometimes removed before cooking.

Hazelnuts are used in baking and desserts, confectionery to make praline, and also used in combination with chocolate for chocolate truffles and products such as chocolate bars, hazelnut cocoa spread such as Nutella, and Frangelico liqueur. Hazelnut oil, pressed from hazelnuts, is strongly flavoured and used as a cooking oil. Turkey is the world's largest producer of hazelnuts.