Polyhedral dice are dice that have some non-standard number of sides (the standard being six). Polyhedral dice first became popular with the advent of role-playing games in the 1970s.
Polyhedral dice are often notated with the convention "NdN," where the first N is the number of dice of a matching size, and the second N the number of sides (the number of sides is what is meant by "size"). For example, 3d6 means 3 cubic dice, 2d8 means 2 octahedral dice, but 1d8+1d12 means roll and add an octahedral and an dodecahedral die. Also, d100 or d% means roll 2 d10s but take the rolls separately as digits instead of adding them. (Some d10s are marked to show they roll the 10s digit. Without one of these, d10s of different colors are rolled after declaring which is the tens digit and which is the 1s).
Usually, a set of polyhedral dice denotes 1d4, 1d6, 1d8, 2d10 (one marked 00, 10, 20,..., 80, 90) 1d12 and 1d20 all in one color. Sometimes a set of polyhedral dice contains only 1d10, or has 3d6.
For a list of games that use these dice, see Category:Polyhedral Dice.