tummy crunch vs sit-up

tummy crunch and sit-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
tummy crunch	Yes	No	No	No
sit-up	Yes	No	No	No

As nouns, sit-up is a hyponym of tummy crunch; that is, sit-up is a word with a more specific, narrower meaning than tummy crunch:

tummy crunch: an exercise designed to strengthen the abdominal muscles
sit-up: a stomach exercise in which a person sits up from a supine position without using the arms for leverage

tummy crunch (noun)	sit-up (noun)
an exercise designed to strengthen the abdominal muscles	a stomach exercise in which a person sits up from a supine position without using the arms for leverage