Used by guys who are turning gay or think they might be gay to make an easy switch from girls to guys. The transition girlfriend is usually extremely butch and masculine and would not get a boyfriend besides a man who thinks he's gay. It's pretty much the manliest woman the man can find. The next step after the transition girlfriend is a very feminine gay guy that is almost like a woman.
Guy 1: "Dude, did you see Mark with that butch chick who's never had a date in her life?"
Guy 2: "Uh-oh dude, I think that's a transition girlfriend!"
Guy 1: "OH GOD NO!!"