debt instrument vs certificate of indebtedness

debt instrument and certificate of indebtedness both are nouns.

Word	Noun	Adjective	Verb	Adverb
debt instrument	Yes	No	No	No
certificate of indebtedness	Yes	No	No	No

As nouns, debt instrument and certificate of indebtedness are synonyms defined as:

debt instrument and certificate of indebtedness: a written promise to repay a debt

Other synonyms of debt instrument include obligation.

debt instrument (noun)	certificate of indebtedness (noun)
a written promise to repay a debt	a written promise to repay a debt