First, to get the definitions out of the way: there are words we use in our community which are different from the ways they are defined by muggles, such as "dominance" or "slavery." "Sadist" is one of those. Like "slavery," the main difference between our usage and the one in the dictionary is that ours assumes mutual informed consent.

Apart from that, the definition is the same: one who takes pleasure in inflicting pain. And yes, brwney, some people do get off on pain, I've been there too. When I first tried bottoming, I tried to handle it as`something I was enduring to please my Domme, which didn't work well. But when I learnt to stop trying to be tough, to sob and beg and accept her right to hurt me, then the pain became part of the joyful experience of submission.

Physically speaking, yes, I am told that experienced pain-sluts can get high on the endorphin rush of pain, the same way that exercise addicts do, but it still needs to be in the right emotional setting - "the right sort of pain," as I've often heard subs say. It is a BDSM cliche that nobody ever got off on stubbing their toe, and I've never heard of anyone who chose not to have pain relief at the dentists' (despite the character in "Little Shop of Horrors.) But when I had a hot female dentist I did occasionally get through some really nasty proceedures by fantasising that I was being tortured by a Domme: it didn't turn me on, but it made it easier to bear.

For me, sadism works the same way. Hurting a sub is enjoyable in exactly the same way that making hir crawl and obey me is enjoyable, they are both expressions of my power over hir. Causing pain is just more direct and physical, like the difference between fucking and exchanging words of love.