portal tomb vs megalith

portal tomb and megalith both are nouns.

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

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

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

Other hypernyms of portal tomb include megalithic structure.

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