Hazelnut and Filbert are fruits of the hazelnut or cobnut bush. They are also called Cob Nuts. Filberts are the cultivated and more robust nuts. Both hazelnuts and filberts are rich in oil, so used in butters, sweets and desserts.
Dowell, P., Bailey, A. (1980) The Book of Ingredients, Dorling Kindersley. ISBN 0718119150.