sweet buckeye vs horse chestnut

sweet buckeye and horse chestnut both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet buckeye	Yes	No	No	No
horse chestnut	Yes	No	No	No

As nouns, horse chestnut is a hypernym of sweet buckeye; that is, horse chestnut is a word with a broader meaning than sweet buckeye:

sweet buckeye: a tall and often cultivated buckeye of the central United States
horse chestnut: tree having palmate leaves and large clusters of white to red flowers followed by brown shiny inedible seeds

Other hypernyms of sweet buckeye include Aesculus hippocastanum, buckeye.

sweet buckeye (noun)	horse chestnut (noun)
a tall and often cultivated buckeye of the central United States	tree having palmate leaves and large clusters of white to red flowers followed by brown shiny inedible seeds
	the inedible nutlike seed of the horse chestnut