Megalithic structure and Portal tomb

Megalithic structure noun - Memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe).

Portal tomb and megalithic structure are semantically related. In some cases you can replace phrase "Portal tomb" with "Megalithic structure", this phrases are similar.

Portal tomb noun - A prehistoric megalithic tomb typically having two large upright stones and a capstone.

Megalithic structure and portal tomb are semantically related. You can use "Megalithic structure" instead a noun phrase "Portal tomb".