what defines a slut?

Well i'm just leaving for work and reading over some of these posts and i'm wondering what people think it takes to be a slut.

Is it a the number of guys a girl has slept with? The time it took her to sleep with them? Do they have to be out of a relationship to count towards her slut factor? Does this also apply to guys?

My ex had slept with some 10guys (1 being me :( ) in 5-6 years, i considered her a slut.

Your thoughts?