Juglans nigra vs walnut

Juglans nigra and walnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
Juglans nigra	Yes	No	No	No
walnut	Yes	No	No	No

As nouns, walnut is a hypernym of Juglans nigra; that is, walnut is a word with a broader meaning than Juglans nigra:

Juglans nigra: North American walnut tree with hard dark wood and edible nut
walnut: any of various trees of the genus Juglans

Other hypernyms of Juglans nigra include walnut tree.

Juglans nigra (noun)	walnut (noun)
North American walnut tree with hard dark wood and edible nut	nut of any of various walnut trees having a wrinkled two-lobed seed with a hard shell
	any of various trees of the genus Juglans
	hard dark-brown wood of any of various walnut trees; used especially for furniture and paneling