It's basic translation is Massively Multiplayer Online. Massively was really a differentiator from Multiplayer which were usually limited to anywhere from 2 to 16 or maybe 32 players at once in an area, as I remember it.
So when people refer to SWTOR as an MMO, I think having 2500 people at once able to be in the same area as massively enough. I'd love it to be larger in scope, but I guess that's up to the optimization and engine teams.
You can't have 2500 people in the same area in this game. You can't even have 100, because the game will spawn multiple instances of the zone and segregate you all. The entire game is about segregating the playerbase into their own little rooms so they can feel like their decisions matter, but they never do.