Definition: A tattoo that is made from UV (ultra-violet) reactive ink or glow-in-the-dark ink inserted under the skin. It is still unknown just how safe these inks are, especially in the long-term, and is strongly advised against. Reputable artists will not do glow tattoos, and for good reason.
At the very least, most of these tattoos look terrible after a period of time. The "ink" gives the skin a horrible discoloration that is unattractive when not in the dark.