greenback vs paper currency

greenback and paper currency both are nouns.

Word	Noun	Adjective	Verb	Adverb
greenback	Yes	No	No	No
paper currency	Yes	No	No	No

As nouns, paper currency is a hypernym of greenback; that is, paper currency is a word with a broader meaning than greenback:

greenback: a piece of paper money (especially one issued by a central bank)
paper currency: currency issued by a government or central bank and consisting of printed paper that can circulate as a substitute for specie

Other hypernyms of greenback include folding money, paper money.

greenback (noun)	paper currency (noun)
a piece of paper money (especially one issued by a central bank)	currency issued by a government or central bank and consisting of printed paper that can circulate as a substitute for specie