silver certificate vs greenback

silver certificate and greenback both are nouns.

Word	Noun	Adjective	Verb	Adverb
silver certificate	Yes	No	No	No
greenback	Yes	No	No	No

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

silver certificate: formerly a bank note issued by the United States Treasury and redeemable in silver
greenback: a piece of paper money (especially one issued by a central bank)

Other hypernyms of silver certificate include bank bill, bank note, banker's bill, banknote, bill, Federal Reserve note, government note, note.

silver certificate (noun)	greenback (noun)
formerly a bank note issued by the United States Treasury and redeemable in silver	a piece of paper money (especially one issued by a central bank)