banknote vs greenback

banknote and greenback both are nouns.

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

As nouns, banknote and greenback are synonyms defined as:

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

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

banknote (noun)	greenback (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)