greenback vs dollar

greenback and dollar both are nouns.

Word	Noun	Adjective	Verb	Adverb
greenback	Yes	No	No	No
dollar	Yes	No	No	No

As nouns, dollar is a hyponym of greenback; that is, dollar is a word with a more specific, narrower meaning than greenback:

greenback (noun)	dollar (noun)
a piece of paper money (especially one issued by a central bank)	a symbol of commercialism or greed
	a piece of paper money worth one dollar
	a United States coin worth one dollar
	the basic monetary unit in many countries; equal to 100 cents