MMOs for which you only pay once after which it's playable like any other game with no need to pay subscription if one even exists. Pay-once bit does not extend to expansions or possible microtransactions.
Name variations: Buy to play
The first video game about Pay-once MMO was released on April 26, 2005.
This is the default for most non-free non-MMO games which is why it's MMO specific.