A blister, callus, or sore you get on the tip of your index finger. Specifically from the repeated tapping, swiping and general touching that occurs while using an iPad, iPhone, or other touch screen (this includes Androids and Microsoft touch screens). Other than putting down the touch device and letting it heal on it's there is no known cure
Ouch, my finger hurts, because I have and iBlister. I really need to break my addiction to candy crush.