I was going to post my definition of "slut", but Theresa did it perfectly... and then MisRed came along and added the next aspect that always is true. A slut is someone who's had more partners than *me* (and that is a general me).
Personally, MY definition of slut is closer to Theresa's. I've been CALLED a slut by folk who used MisRed's caveat. So eh.