megalith

A very large stone used in various prehistoric architectures or monumental styles, notably in western Europe during the second millennium BC.

A great stone; specifically, a stone of great size used in constructive work or as a monument, as in ancient Cyclopean and so-called Druidic or Celtic remains.

A large stone; especially, a large stone used in constructing ancient monuments.

A construction involving one or several roughly hewn stone slabs of great size.

A large stone used in such a construction.

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