wild card

Concise Oxford English Dictionary © 2008 Oxford University Press:
wild card

▶noun

• 1 a playing card that can have any value, suit, colour, or other property in a game at the discretion of the player holding it.

  ■ Computing a character that will match any character or sequence of characters in a search.

• 2 a person or thing whose qualities are uncertain.

• 3 an opportunity to enter a sports competition without taking part in qualifying matches or being ranked at a particular level.

  ■ a player or team given such an opportunity.