common buttercup vs buttercup

common buttercup and buttercup both are nouns.

Word	Noun	Adjective	Verb	Adverb
common buttercup	Yes	No	No	No
buttercup	Yes	No	No	No

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

common buttercup: perennial Old World buttercup with golden to sulphur yellow flowers in late spring to early summer; naturalized in North America
buttercup: any of various plants of the genus Ranunculus

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

common buttercup (noun)	buttercup (noun)
perennial Old World buttercup with golden to sulphur yellow flowers in late spring to early summer; naturalized in North America	any of various plants of the genus Ranunculus