citrous fruit vs lemon

citrous fruit and lemon both are nouns.

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

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

citrous fruit: any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions
lemon: yellow oval fruit with juicy acidic flesh

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

citrous fruit (noun)	lemon (noun)
any of numerous fruits of the genus Citrus having thick rind and juicy pulp; grown in warm regions	an artifact (especially an automobile) that is defective or unsatisfactory
	a strong yellow color
	a distinctive tart flavor characteristic of lemons
	yellow oval fruit with juicy acidic flesh
	a small evergreen tree that originated in Asia but is widely cultivated for its fruit