The Collaborative International Dictionary of English v.0.48:
Megalith \Meg"a*lith\, n. [Mega- + -lith; cf. F. m['e]galithe.] A large stone; especially, a large stone used in constructing ancient monuments. -- Meg`a*lith"ic, a. [1913 Webster]WordNet (r) 3.0 (2006):
megalith n 1: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe) [syn: megalith, megalithic structure]