A primitive, unwritten, unspoken, totally subjective, ever-changing rule that defines how much a girl can slut herself up without becoming a target for harsh criticism from her female peers.
As it doesn't stem from any higher brain function, the subjective usually (possibly truthfully) denies all knowledge, though mentioning it will usually provoke other areas of the reptilian part of the brain, typically the medula oblongata, resulting in a burst of anger.
As old as time itself, this rule continues to be prodded and probed by 'mostly' young women, trying to stretch the boundaries and hence gain the upper hand in the popularity-with-boys struggle. This continued activity has seen the rule become extremely lax over the last few decades.
Psychologists also refer to this primitive ideology as the "Alpha Male Competition for Women" as popularity with the opposite gender is the most common way that women gauge their own, and each other's success in the world.
Sofie: Awww I hate that bloody Janie. She's such a slut! Look at that thing she's wearing.
Bubbles: Yeah! Whadda slut! That's not a skirt, it's a belt! (... Gee I'm glad I didn't wear my really short mini)
Bloke: G'day gals, whatcha drinking? Four-X or VB? 'Ang on, who's that hottie?! (Walks over to Janie)
Sofie & Bubbles: Awww that SLUT!!
Bubbles: ... Sofie, what's with your makeup tonight anyway?
Sofie: Shut up! What's with those slutty shoes!?
Bubbles: Awww you Slut!
Bloke to Janie: Awww, looks like someone violated the Slut-trust.
Janie: Fuck off you bastard! Womens Lib!