Any piercing through the between-the-fingers webbing.
Hand web piercings are most commonly rings between the thumb and forefinger, but are also often placed between any fingers.
Contrary to urban legend, this piercing is not at all dangerous. Perhaps the only additional risk is that your hands come in contact with more foreign bacteria than any other part of your body. Additional care needs to be taken to keep everything clean. As well as regular cleaning, salt soaks are essential.
To minimize rejection, placement is essential. An experienced piercer will place the jewelry so that it moves minimally and irritates the exit holes as little as possible. In addition, you may find that taping the surrounding fingers reduces the stresses that everyday movements put on the exit holes.