stomach ache vs stomachache

stomach ache and stomachache both are nouns.

Word	Noun	Adjective	Verb	Adverb
stomach ache	Yes	No	No	No
stomachache	Yes	No	No	No

As nouns, stomach ache and stomachache are synonyms defined as:

stomach ache and stomachache: an ache localized in the stomach or abdominal region

Other synonyms of stomach ache include bellyache, gastralgia.

stomach ache (noun)	stomachache (noun)
an ache localized in the stomach or abdominal region	an ache localized in the stomach or abdominal region