promise vs rain check

promise and rain check both are nouns.

Word	Noun	Adjective	Verb	Adverb
promise	Yes	No	Yes	No
rain check	Yes	No	No	No

As nouns, rain check is a hyponym of promise; that is, rain check is a word with a more specific, narrower meaning than promise:

promise: a verbal commitment by one person to another agreeing to do (or not to do) something in the future
rain check: a promise that an unaccepted offer will be renewed in the future

Other hyponyms of promise include oath, parole, word, word of honor, betrothal, engagement, troth, pinning.

promise (noun)	rain check (noun)
grounds for feeling hopeful about the future	a ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain
a verbal commitment by one person to another agreeing to do (or not to do) something in the future	a promise that an unaccepted offer will be renewed in the future

promise (verb)	rain check (verb)
make a promise or commitment
promise to undertake or give
make a prediction about; tell in advance
give grounds for expectations