citrus fruit vs citron

citrus fruit and citron both are nouns.

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

As nouns, citron is a hyponym of citrus fruit; that is, citron 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
citron: large lemonlike fruit with thick aromatic rind; usually preserved

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

citrus fruit (noun)	citron (noun)
any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions	large lemonlike fruit with thick aromatic rind; usually preserved
	thorny evergreen small tree or shrub of India widely cultivated for its large lemonlike fruits that have thick warty rind