TENS unit is an acronym for Transcutaneous Electrical Nerve Stimulation Unit. TENS units are prescribed by physicians and physical therapists to control pain. Mild electrical currents are delivered through wires to electrodes adhered to the skin. Electrodes are usually self-adhesive and are placed near the site of pain. It is not known why the electrical stimulation aids in pain reduction. It has been hypothesized that the electrical current may release endorphins in the body. Endorphis are the body's natural pain killers.
In the context of body modification, the TENS unit is a tool used in Electrical Play. The electrodes are placed on the genitals or other areas of the body for the purpose of sexual gratification. The TENS unit may also be used by a Sadist on their willing sexual partner for the purpose of inflicting fear more than physical pain.