megalith

noun

1. a stone of great size, especially in ancient construction work, as the Cyclopean masonry, or in prehistoric Neolithic remains, as dolmens or menhirs.

noun

1. a stone of great size, esp one forming part of a prehistoric monumentSee also alignment (def. 6), circle (def. 11)

n.huge prehistoric stone, 1853, back-formation from megalithic.