tall buttercup vs kingcup

tall buttercup and kingcup both are nouns.

Word	Noun	Adjective	Verb	Adverb
tall buttercup	Yes	No	No	No
kingcup	Yes	No	No	No

As nouns, kingcup is a hypernym of tall buttercup; that is, kingcup is a word with a broader meaning than tall buttercup:

tall buttercup: perennial European buttercup with yellow spring flowers widely naturalized especially in eastern North America
kingcup: any of various plants of the genus Ranunculus

Other hypernyms of tall buttercup include butter-flower, buttercup, butterflower, crowfoot, goldcup.

tall buttercup (noun)	kingcup (noun)
perennial European buttercup with yellow spring flowers widely naturalized especially in eastern North America	any of various plants of the genus Ranunculus
	swamp plant of Europe and North America having bright yellow flowers resembling buttercups