As I understand it, a monolith is a single stone (mono). A Megalith, though, is a monument from the Megalithic period as opposed to mega in size. So it could be a cromlech, monolith, portal tomb, etc. Possibly. In fact don't ask me because I always get confused between tumulae and barrows and other types of tombs! I've got a great prehistoric dictionary too, maybe I should stop posting and making a fool and go and find it....