greenback vs bank bill

greenback and bank bill both are nouns.

Word	Noun	Adjective	Verb	Adverb
greenback	Yes	No	No	No
bank bill	Yes	No	No	No

As nouns, greenback and bank bill are synonyms defined as:

greenback and bank bill: a piece of paper money (especially one issued by a central bank)

Other synonyms of greenback include bank note, banker's bill, banknote, bill, Federal Reserve note, government note, note.

greenback (noun)	bank bill (noun)
a piece of paper money (especially one issued by a central bank)	a piece of paper money (especially one issued by a central bank)