Source : Webster's Revised Unabridged Dictionary (1913)

Megalith \Meg"a*lith\, n. [Mega- + -lith; cf. F. m['e]galithe.]
   A large stone; especially, a large stone used in ancient
   building. -- {Meg`a*lith"ic}, a.

Source : WordNet®

megalith
     n : memorial consisting of a very large stone forming part of a
         prehistoric structure (especially in western Europe)
         [syn: {megalithic structure}]