Megalith and Portal tomb

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

Portal tomb and megalith are semantically related. In some cases you can replace phrase "Portal tomb" with "Megalith", this terms are similar.

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

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