The girl you commonly hang out with while in-game of your favorite massively multi-player online role-playing game. Her irl boyfriend plays an MMORPG, but she doesn't like the one he plays, so she happens to play the one you like. This results in you and your MMO Girlfriend spending more time with each other than with your irl relationships. You are either single irl, or your irl girlfriend/wife does not play MMOG's, in which case you will soon be single irl. You are not allowed by your MMO Girlfriend to complete quests or advance level without her, so you will likely create at least one other alternate toon to play while she is offline.
To your friends in Vent, this relationship is especially comical if you chose to make your in-game character a female, and your MMO Girlfriend chose to make her toon a male. This is actually a very common occurrence, due to the male player's desire to create an attractive toon to stare at, and the female player's desire to remain largely unidentifiable as a female irl.