head ache vs headache

head ache and headache both are nouns.

Word	Noun	Adjective	Verb	Adverb
head ache	Yes	No	No	No
headache	Yes	No	No	No

As nouns, head ache and headache are synonyms defined as:

head ache and headache: pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs

Other synonyms of head ache include cephalalgia.

head ache (noun)	headache (noun)
pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs	something or someone that causes anxiety; a source of unhappiness
	pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs