A TENS (Transcutaneous Electrical Nerve Stimulation) unit is a small, portable, battery-powered device that uses low-voltage electrical currents to provide pain relief. It works by sending impulses through electrodes placed on the skin near the source of pain, which can block pain signals to the brain or trigger the release of the body's natural painkillers, such as endorphins. The intensity, frequency, and duration of the electrical pulses can be adjusted by the user, and it is used for a variety of acute and chronic pain conditions, including osteoarthritis, back pain, and fibromyalgia.