megalith vs menhir

megalith and menhir both are nouns.

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

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

megalith: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)
menhir: a tall upright megalith; found primarily in England and northern France

Other hyponyms of megalith include cromlech, dolmen, portal tomb, standing stone.

megalith (noun)	menhir (noun)
memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)	a tall upright megalith; found primarily in England and northern France