megalithic structure vs megalith

megalithic structure and megalith both are nouns.

Word	Noun	Adjective	Verb	Adverb
megalithic structure	Yes	No	No	No
megalith	Yes	No	No	No

As nouns, megalithic structure and megalith are synonyms defined as:

megalithic structure and megalith: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)

megalithic structure (noun)	megalith (noun)
memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)	memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)