voucher vs coupon

voucher and coupon both are nouns.

Word	Noun	Adjective	Verb	Adverb
voucher	Yes	No	No	No
coupon	Yes	No	No	No

As nouns, voucher and coupon are synonyms defined as:

voucher and coupon: a negotiable certificate that can be detached and redeemed as needed

voucher (noun)	coupon (noun)
a negotiable certificate that can be detached and redeemed as needed	a test sample of some substance
a document that serves as evidence of some expenditure	a negotiable certificate that can be detached and redeemed as needed
someone who vouches for another or for the correctness of a statement