promissory note vs obligation

promissory note and obligation both are nouns.

Word	Noun	Adjective	Verb	Adverb
promissory note	Yes	No	No	No
obligation	Yes	No	No	No

As nouns, obligation is a hypernym of promissory note; that is, obligation is a word with a broader meaning than promissory note:

promissory note: a promise to pay a specified amount on demand or at a certain time
obligation: a written promise to repay a debt

Other hypernyms of promissory note include certificate of indebtedness, debt instrument.

promissory note (noun)	obligation (noun)
a promise to pay a specified amount on demand or at a certain time	the social force that binds you to the courses of action demanded by that force
	a legal agreement specifying a payment or action and the penalty for failure to comply
	a written promise to repay a debt
	a personal relation in which one is indebted for a service or favor
	the state of being obligated to do or pay something