Quote Originally Posted by Sandpark View Post
Just means you are online the same time as many people in the same world.
If you want to be that technical, you don't even have to all be in the same world. It is sufficient if there are many (Massive) players that can potentially somehow connect to each other (Multiplayer) via the internet (Online).

There doesn't even have to be a persistent world, if you are technical about the term. League of Legends would qualify, as MMOBA.