( Noun) One who finds deep emotional fulfillment in causing measurable, controlled, calculated pain. Implies both those who seek consensual arrangements as well as those who have no concern for agreement on the part of the one being put into pain.
Person who likes to inflict pain on others and gets sexual excitement from it. There are different type of sadists. Beware, not to confuse a bdsm enthusiast with a person who just wants to beat someone else in a non consensual setting... such as a wife beater.