I asked myself the same question... but I realized there are others MMO's out there that only work with "small" servers, and don't necessarily have one huge persistent world à la WoW. The term 'massively' multiplayer could mean two things: it's played in multiplayer a lot (i.e. no single player mode), or that the interactions happen between "massive" amounts of players. But since the limit of players in an arena or a zone is purely theoric, and not something like 16 or 32 players like in most other games that can be played in multiplayer, I guess it is a MMO Battlefield 2 is a multiplayer game because it has a player limit. MMO = no actual set limit (massively is the key word)