round kumquat vs cumquat

round kumquat and cumquat both are nouns.

Word	Noun	Adjective	Verb	Adverb
round kumquat	Yes	No	No	No
cumquat	Yes	No	No	No

As nouns, cumquat is a hypernym of round kumquat; that is, cumquat is a word with a broader meaning than round kumquat:

round kumquat: shrub bearing round-fruited kumquats
cumquat: any of several trees or shrubs of the genus Fortunella bearing small orange-colored edible fruits with thick sweet-flavored skin and sour pulp

Other hypernyms of round kumquat include kumquat, kumquat tree.

round kumquat (noun)	cumquat (noun)
shrub bearing round-fruited kumquats	any of several trees or shrubs of the genus Fortunella bearing small orange-colored edible fruits with thick sweet-flavored skin and sour pulp