A term describing a person with the unusual condition of deriving sexual pleasure from infliction of physical and psychological pain. This kind of arousal feels very different from a romantic one because different parts of the brain cause the arousal -- the parts of the brain that regulate sympathy and predatory desires of domination, control, and the desire to violently exploit perceived weaknesses in a human being.
Sadistic pleasure is very addicting (as most irregular pleasures are). Since the individual will keep using sadistic means of creating sexual satisfaction, he/she can become numb to enjoying normal romantic arousal. This can significantly change their lifestyle, as they might have less motivation to engage in activities that lead to normal sex. In other words, they can become reclusive and weird. The sadistic individual always has to deal with the fact that 99.9% of humans don't like being hurt, and that's a problem. The sadistic individual hears and sees their victims cry in pain. Over time, this feedback has a damaging effect and can lead to depression.
The early conditions of the childhood have a lot to do with becoming a sadistic adult. It's still not yet completely understood how. Somehow, the brain connects sexuality with violence during these early ages.
The sadistic master likes to whip and humiliate his slave. This gives him an erection.