Waterboarding is a form of torture in which water is poured over a cloth covering the face and breathing passages of an immobilized captive, causing the person to experience drowning. In the most common method of waterboarding, the captive's face is covered with cloth or some other thin material and immobilized on his/her back at an incline of 10 to 20 degrees. Torturers pour water onto the face over the breathing passages, causing an almost immediate gag reflex and creating a drowning sensation for the captive. Normally, water is poured intermittently to prevent death. However, if the water is poured uninterruptedly it will lead to death by asphyxia, also called dry drowning. Waterboarding can cause extreme pain, damage to lungs, brain damage from oxygen deprivation, other physical injuries including broken bones due to struggling against restraints, and lasting psychological damage. It is obvious that the technique should only be used in consensual situations, and after agreeing on a safe signal to immediately stop the torture.
Water dunking is a form of corporal punishment used in the medieval and Early Modern (17th-18th century) period. While witches were commonly tortured using this method, thieves and murderers could be subject to it in order to extract a confession. This was more common when other more sophisticated torture devices were not present.
In BDSM terms we use the term for any activity involving the submersion of the head of the sub into water. This can be a bucket, a bath, streaming water in nature, with or without the use of restraints to enhance the feeling of helplessness.