Cocked dice are dice that have been rolled but have not come to rest in a stable position on a flat surface, meaning the result is considered inadmissable in a game and they need to be re-rolled.
Different groups determine whether a dice is cocked in different ways. One trick to determine whether a die is cocked is to try to rest another die on top; if it stays on by itself, the die is not cocked, but if it falls off it is. Some groups take a zero-tolerance approach, requiring that any dice resting at an angle or across two surfaces be re-rolled.
Using game aids like dice towers helps to prevent cocked dice.