stomachache vs stomach ache

stomachache and stomach ache both are nouns.

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

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

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

Other synonyms of stomachache include bellyache, gastralgia.

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