From The Free On-line Dictionary of Computing (19 January 2023):
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)