Editor/usage note: In recent years (2012 onward, possibly even earlier), MMO has become a term used to refer to simply multiplayer games, thus its usage is often very misleading and vague.

The term "massively multiplayer online role-playing game" (MMORPG) was coined by Richard Garriott of Ultima fame. This style of game was formally called "graphical MUDs".

Related:
* MUD
* Clans

Mandatory traits:
* Persistent world - world continues to operate even when the player disconnects and is there when they reconnect the next day, changes made by the players are somewhat permanent.
* Hundreds of concurrent players on same server/shard where they can interact with each other. - not when they're separated into instances of 4-32 players each, even if on same server.
If either of the above is missing, then it's not an MMO.

Traits that indicate it is NOT an MMO: (not exclusive as MMOs may have extra modes where these are present, but they're really good indicators)
* Player lobby
* Matchmaking