I think both Super Healer and Thorsten are on the right track. More than just looking at the individual words that form "MMO" though, you also need to look at the words together.

For example, someone might construe that a game is "massive" because it has thousands of players playing it online in multiplayer groups. Yet if those groups consist of 2 vs 2 combat (i.e. arena), I'd hardly call that "massively multiplayer". This is where I think it gets hard to draw the line.

For example, what if those 2 vs 2 combats were all connected in real time somehow. So ya they are instanced 2 vs 2 sessions but all them are being tracked and their results effect the determination of territorial control in the game in "real time". So yes the overall battle is "massive" but the experience for the player is still just 2 v 2 (so not "massive").

For me, an MMO really has to have a "massive multiplayer" experience to it where more than 64 players are "capable" of fighting in the same combat next to each other (thus greater than a FPS server based gameplay).

The other thing that's not in the MMO name though but has always been implied is "persistence". But even with this aspect, where is the line drawn? For example, does the persistence have to be "forever" (seems logically considering persistence would imply that) or can there be wars that last for days but they eventually have a victor and then everything is reset for the next war? For example, Warbirds is a WWII flight sim game and the battles are massive with many players all fighting towards territorial control. Yet eventually one side wins the war and the world map get reset. To me, this game is definitely a "massively multiplayer online" experience without a doubt and there is even some persistence with regards to territorial control, yet at the same time, its definitely not an MMO like WoW.