Corylus avellana vs hazelnut

Corylus avellana and hazelnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
Corylus avellana	Yes	No	No	No
hazelnut	Yes	No	No	No

As nouns, hazelnut is a hypernym of Corylus avellana; that is, hazelnut is a word with a broader meaning than Corylus avellana:

Corylus avellana: small nut-bearing tree much grown in Europe
hazelnut: any of several shrubs or small trees of the genus Corylus bearing edible nuts enclosed in a leafy husk

Other hypernyms of Corylus avellana include hazel, hazelnut tree.

Corylus avellana (noun)	hazelnut (noun)
small nut-bearing tree much grown in Europe	nut of any of several trees of the genus Corylus
	any of several shrubs or small trees of the genus Corylus bearing edible nuts enclosed in a leafy husk