citrus fruit vs kumquat

citrus fruit and kumquat both are nouns.

Word	Noun	Adjective	Verb	Adverb
citrus fruit	Yes	No	No	No
kumquat	Yes	No	No	No

As nouns, kumquat is a hyponym of citrus fruit; that is, kumquat is a word with a more specific, narrower meaning than citrus fruit:

citrus fruit: any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions
kumquat: small oval citrus fruit with thin sweet rind and very acid pulp

Other hyponyms of citrus fruit include orange, mandarin, mandarin orange, lemon, lime, grapefruit, pomelo, shaddock, citrange, citron, calamondin.

citrus fruit (noun)	kumquat (noun)
any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions	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