headstone vs monument

headstone and monument both are nouns.

Word	Noun	Adjective	Verb	Adverb
headstone	Yes	No	No	No
monument	Yes	No	No	No

As nouns, monument is a hypernym of headstone; that is, monument is a word with a broader meaning than headstone:

Other hypernyms of headstone include memorial, stone.

headstone (noun)	monument (noun)
the central building block at the top of an arch or vault	a structure erected to commemorate persons or events
a stone that is used to mark a grave	a burial vault (usually for some famous person)
	an important site that is marked and preserved as public property