MMO
Also said as MMORPG, Massively multiplayer online game
A persistent online world shared with thousands of players at once, which carries on when you log off. It promises a place, and a society, more than a story: other people are the content.
Other people give you the economy, the groups and the status that a solo game has to fake. The cost is servers and a live team for as long as it runs, and an empty MMO feels dead.
Say it
“This MMO's first world boss should need about 20 players and spawn every two hours, so that word spreads, people gather and guilds have a reason to exist.”
Where you’ve seen it
Ultima Online (1997) and EverQuest (1999) set up the form, and World of Warcraft (2004) brought it to millions. Its first big raid, Molten Core, took 40 players at once, which no one could do alone. EVE Online keeps all its players on a single shard.
Not to be confused with online game
A match of an online shooter ends and its world disappears. An MMO's world keeps going, and what you did in it remains.