first stomach vs tummy

first stomach and tummy both are nouns.

Word	Noun	Adjective	Verb	Adverb
first stomach	Yes	No	No	No
tummy	Yes	No	No	No

As nouns, tummy is a hypernym of first stomach; that is, tummy is a word with a broader meaning than first stomach:

first stomach: the first compartment of the stomach of a ruminant; here food is collected and returned to the mouth as cud for chewing
tummy: an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion

Other hypernyms of first stomach include breadbasket, stomach, tum.

first stomach (noun)	tummy (noun)
the first compartment of the stomach of a ruminant; here food is collected and returned to the mouth as cud for chewing	an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion
	slang for a paunch