citron vs citrous fruit

citron and citrous fruit both are nouns.

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

As nouns, citrous fruit is a hypernym of citron; that is, citrous fruit is a word with a broader meaning than citron:

citron: large lemonlike fruit with thick aromatic rind; usually preserved
citrous fruit: any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions

Other hypernyms of citron include citrus, citrus fruit.

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