megalith vs monument

megalith and monument both are nouns.

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

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

megalith: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)
monument: a structure erected to commemorate persons or events

Other hypernyms of megalith include memorial.

megalith (noun)	monument (noun)
memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)	a structure erected to commemorate persons or events
	a burial vault (usually for some famous person)
	an important site that is marked and preserved as public property