buckthorn vs black haw

buckthorn and black haw both are nouns.

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

As nouns, black haw is a hyponym of buckthorn; that is, black haw is a word with a more specific, narrower meaning than buckthorn:

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