white walnut vs walnut

white walnut and walnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
white walnut	Yes	No	No	No
walnut	Yes	No	No	No

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

white walnut: North American walnut tree having light-brown wood and edible nuts; source of a light-brown dye
walnut: any of various trees of the genus Juglans

Other hypernyms of white walnut include walnut tree.

white walnut (noun)	walnut (noun)
North American walnut tree having light-brown wood and edible nuts; source of a light-brown dye	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