The AD&D abbreviation for a twelve-sided die. It's a dodecahedron with faces labelled 1 - 12. 1d12 means "roll one twelve-sided die once", 3d12+3 means "roll three twelve-sided dice, add the results, and then add three to that", and so forth. Not used a whole lot compared with the other dice. The d8, d6, and d10 are much more popular.