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®

megalithic
     adj : of or relating to megaliths or the people who erected
           megaliths; "megalithic monuments like Stonehenge"