How to name the fact that for a MMO game there's one big session, the same things are happening for all the players, you log out and after two days log in to the same place as the same person ?
I'd like to mention that not all MMO games are constructed this way, for example majority of MMOFPS games are based on lobby/separated severs system, so prefix "MMO" isn't necessarily equal to "onesessionism".
I myself can't think of any better expression than "permanent world".