JOKER

[noun] [C] - a special playing card which can be given any value and used in some card games instead of any other cardThe joker does not belong to any of the four suits in cards: hearts, diamonds, clubs or spades.See also joker at joke (AMUSING), joke (RIDICULOUS).If you say that someone or something is the joker in the pack, you mean that they are the person or thing that could change the situation in an unexpected way.