A female who is used by a male to temporarily fill the position of "girlfriend" until a legitimate relationship opportunity presents itself.
The stand-in gf is kept around for the amusement and satisfaction of the male, and so he doesn't get lonely.
The male will often spew phrases such as "I love you" and "I love my girlfriend", though it is and always was his intent to drop said female for one who is actually attractive.
See also: stand-in boyfriend
Chick 1: Check out that ugly hag hanging out with that hottie. He's way out of her league!
Chick 2: Yea she's definitely a stand-in girlfriend, I'm gonna go talk to him.