black haw vs buckthorn

black haw and buckthorn both are nouns.

Word	Noun	Adjective	Verb	Adverb
black haw	Yes	No	No	No
buckthorn	Yes	No	No	No

As nouns, buckthorn is a hypernym of black haw; that is, buckthorn is a word with a broader meaning than black haw:

black haw (noun)	buckthorn (noun)
upright deciduous shrub having frosted dark-blue fruit; east and east central North America	any shrub or small tree of the genus Bumelia
deciduous tree of southeastern United States and Mexico	a shrub or shrubby tree of the genus Rhamnus; fruits are source of yellow dyes or pigments
	an Old World plantain with long narrow ribbed leaves widely established in temperate regions