A guy you meet at a concert or music festival that you dance (or whatever) with for one song or even the whole time you're there. Usually you pick a guy you've never seen and will probably never see again. Sometimes relationships or friendships can form. Sometimes this is more like a one night (concert) stand situation.
(At a concert)
Girl 1: Anyone see any potential concert boyfriends?
Girl 2: Right over there, I've never seen them around here lets go see if they want to be our boyfriends for tonight.
Girl 1: I like the brunette, maybe I'll even give him my number.