A d12, or twelve-sided die, is a die that has an equal chance of rolling the integers from 1 to 12. Its most common form is a regular dodecahedron, in which form it appears in the standard 7-dice set used in role-playing games like Dungeons & Dragons.
The d12 is one of the less-frequently used dice in the standard set, partly because 2d6 can generate a similar range of outcomes with more easily accessible dice. However, d12s are one of the best dice for rolling, and twelve-sided d4s and d6s (in which the relevant numbers are repeated to fill the available faces) are sometimes used by players instead of the standard 4-sided and 6-sided varieties.
Even in Dungeons & Dragons, there are relatively few uses for d12s. Certain large weapons in some editions of the game use the d12 to calculate damage, and the barbarian class uses the d12 for their hit dice in later editions of the game. They are very rarely used in magic spells or other contexts.
- d12 at White Wolf Wiki