I think the def. is clear. Massivly Multiplayer Onlinegame. And that it is. I think an MMO normaly just needs Worlds to put the 'x # of players' somewhere. But if that is not required, because the game has got an different concept, it' OK.
What you maybe mean is that there's interaction between gamers missing, which normally takes place in a kind of 'world'.
But there are also games which have worlds but the interaction is missing.!!!!!!!!!