A fair number cube is a cube that has six sides, typically each displaying a different number. Fair number cubes are mostly used to generate random numbers in an equal manner, meaning each number has an equal, or a one in six, chance of being rolled.
Fair number cubes are usually associated with probability. The theoretical probability is that each face has 1/6 chance of being rolled, meaning that each of the six faces is equally likely to turn up. These cubes can also be used in probability questions, such as the probability of either a 2 or a 4 being rolled. In this case, the answer is a one in three chance.