The hazelnut is the fruit of the hazel tree 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
Ripe hazelnuts
Cracked hazelnut shell displaying the edible seed
Hazelnut tree, Turkey
Hand-harvest and sun-drying of hazelnuts in Turkey
A nut is a fruit consisting of a hard or tough nutshell protecting a single seed. In botany, the use of the term nut implies that the shell does not open to release the seed.
Hazelnut
…The hazelnut is the fruit of the hazel tree and therefore includes any of the nuts deriving from species of the genus Corylus, especially the nuts of the species Corylus avellana…
Diagram of a shagbark hickory nut
An Eastern chipmunk (Tamias striatus) with a hazelnut in its mouth