tummy vs internal organ

tummy and internal organ both are nouns.

Word	Noun	Adjective	Verb	Adverb
tummy	Yes	No	No	No
internal organ	Yes	No	No	No

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

tummy: an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion
internal organ: a main organ that is situated inside the body

Other hypernyms of tummy include viscus.

tummy (noun)	internal organ (noun)
an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion	a main organ that is situated inside the body
slang for a paunch