Fortunella japonica vs kumquat

Fortunella japonica and kumquat both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fortunella japonica	Yes	No	No	No
kumquat	Yes	No	No	No

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

Fortunella japonica: shrub bearing round-fruited kumquats
kumquat: 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 Fortunella japonica include cumquat, kumquat tree.

Fortunella japonica (noun)	kumquat (noun)
shrub bearing round-fruited kumquats	small oval citrus fruit with thin sweet rind and very acid pulp
	any of several trees or shrubs of the genus Fortunella bearing small orange-colored edible fruits with thick sweet-flavored skin and sour pulp