megalith vs portal tomb

megalith and portal tomb both are nouns.

Word	Noun	Adjective	Verb	Adverb
megalith	Yes	No	No	No
portal tomb	Yes	No	No	No

As nouns, portal tomb is a hyponym of megalith; that is, portal tomb 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)
portal tomb: a prehistoric megalithic tomb typically having two large upright stones and a capstone

Other hyponyms of megalith include cromlech, dolmen, menhir, standing stone.

megalith (noun)	portal tomb (noun)
memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)	a prehistoric megalithic tomb typically having two large upright stones and a capstone