MMO means Massively Multiplayer Online.[1] Star Citizen is an MMO, while Squadron 42 isn't.
The term MMO was coined at Origin Systems by Richard Gariott's team trying to differentiate Ultimate Online from MUD’s and other dial up service multiplayer games.[1]
Foremost, was the concept of “massive”, meaning that truly thousands or more players could simultaneously connect and interact with each other live in the game world. [1]
As Ultima Online had significantly more success than Electronic Access had projected, they still had to “shard” the world.[1]
While Ultima Online was mostly a single flat world, Origin Systems did not intend to imply that the massive number of players in an MMO could not also have smaller instanced spaces, rather / mostly that all players in a shard can see and interact with each other.[1]