An abbreviated term for "two six-sided dice". This form of abbreviation is common shorthand for specifying the number and type of dice to be used in a specific roll. Most commonly found in tabletop role-playing games, though the format is sometimes found in boardgames. These abbreviations always have the number of dice first, followed by the D (indicating that this is a dice roll), followed by the number of sides each die should have. 2D6 is one of the most common rolls, especially in boardgames; that's why I've noded it.