When a guy has an unhealthy obsession with a girl, particularly one who does not love him back, but that obsession disappears the moment he sees another pretty girl and becomes obsessed with her instead.
Named after Romeo from Shakespeare's Romeo and Juliet, because Romeo was in love with a girl named Rosaline, but forgot all about her the moment he saw Juliet.
Jason is obsessed with a new girl every few weeks. He has such a Romeo complex.