citrus fruit vs pomelo

citrus fruit and pomelo both are nouns.

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

As nouns, pomelo is a hyponym of citrus fruit; that is, pomelo 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
pomelo: large pear-shaped fruit similar to grapefruit but with coarse dry pulp

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

citrus fruit (noun)	pomelo (noun)
any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions	large pear-shaped fruit similar to grapefruit but with coarse dry pulp
	southeastern Asian tree producing large fruits resembling grapefruits