Torture is the act of intentionally causing extreme pain or suffering, physical or mental, on a creature, usually a human. Torture has been around since prehistoric times, and has primarily been used as an interrogation technique for extracting intelligence from an uncooperative subject. The motive for torture can also be for the sadistic gratification of the torturer.
Torture is primarily caused by causing physical pain to the target. Traditionally, the subject is restrained, or otherwise unable to resist the torturer. The torturer then can use nearly any technique of corporal punishment, although to a more extreme degree. Bondage is a common element, as is humiliation. All manners of instruments are used, including some that would be too dangerous for corporal punishment. Occasionally, sexual violence is used as a weapon, although almost exclusively against females.
"Torture" in BDSM
Die Folterkammer des Masochisten" (The Masochist's Torture Chamber), by R. Planitz