D20
From the White Wolf Wiki, the most complete White Wolf reference
A d20 — also called a twenty-sided die or, less commonly, an icosahedron — is one of several standard polyhedral dice used in gaming. Each type is generally identified by the use of the letter "d" (for "die" or "dice") and the number of sides the die features. Multiple dice are connotated by an additional number prefixing the D. So rolling five twenty-sided dice would be noted by writing 5d20.
The d20 is the most important die in the d20/OGL system, and is used for most rolls in the game.
