Personally I agree most with DawnOwl. To me, a slut (when you're referred to as such in a Dom/sub setting - on this board, there's really no point discussing what 'slut' means when it's not in this sort of setting, is there?) is someone who just can't stop thinking about sex and being used for sex, regardless of where he/she is, the time of day, and whether the Dom is present or not. Sluts could turn even the most innocent of everyday situations (like passing by a big strong man, being scolded by the boss, being patted on the arm by a co-worker, etc.) into a sexual/humiliating one in their heads. They also respond to readily sexual situations (like when their Dom tells them to do something) with enthusiasm and sometimes by doing more than they're expected to, and when their Doms say "good little slut", they revel in the feeling of pride+depravity it gives them.

Well, that's my take on it, anyway. Feel free to disagree.. :P