ache vs head ache

ache and head ache both are nouns.

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

As nouns, head ache is a hyponym of ache; that is, head ache is a word with a more specific, narrower meaning than ache:

ache: a dull persistent (usually moderately intense) pain
head ache: pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs

ache (noun)	head ache (noun)
a dull persistent (usually moderately intense) pain	pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs

ache (verb)	head ache (verb)
have a desire for something or someone who is not present
be the source of pain
feel physical pain