rich man vs nabob

rich man and nabob both are nouns.

Word	Noun	Adjective	Verb	Adverb
rich man	Yes	No	No	No
nabob	Yes	No	No	No

As nouns, nabob is a hyponym of rich man; that is, nabob is a word with a more specific, narrower meaning than rich man:

Other hyponyms of rich man include nob, toff.

rich man (noun)	nabob (noun)
a man who is wealthy	a wealthy man (especially one who made his fortune in the Orient)
	a governor in India during the Mogul empire