A style of martial arts that uses pressure points. Note that many other styles use pressure points too, but kyusho jitsu focuses on them from the beginning whereas in other styles they are taught only for advanced students due to the hazardous nature (the students have to be adults AFAIK). Pressure points are used to create pain, which is helpful in self-defense, or to knock out the subject. Usually these techniques don't have as hard effect as a regular brutal beat up, and are therefore easier to justice.
The founder of its current (and/or westernised) form was George A. Dillman of Dillman Karate International (who by the way has trained with both Bruce Lee and Muhammad Ali) who sought more info about the "secret" pressure point techniques of the Japanese. He has also done research on kata and their deeper meanings.