The Free On-line Dictionary of Computing (30 December 2018):

Massively Multiplayer Online Game
Massively Multiplayer Online Role-Playing Game
MMO
MMOG
MMORPG

   (MMOG, MMO) Any game that allows dozens, hundreds or even
   thousands of players to interact with a game via the
   Internet.  Typically the game runs on a central server
   farm and players access it via a personal computer, game
   console or mobile phone.  The most popular genre is the
   Massively Multiplayer Online Role Playing Game (MMORPG), of
   which World of Warcraft is probably the most popular
   example.

   Note that an MMOG is not necessarily a _massive_ game (though
   often they are based in large, complex worlds), their
   distinguishing characteristic is the number of players.

   (2012-05-30)