wild card

noun

• a playing card that can have any value, suit, color, or other property in a game at the discretion of the player holding it.
• a person or thing whose influence is unpredictable or whose qualities are uncertain.
• Computing a character that will match any character or sequence of characters in a search.
• an opportunity to enter a sports competition without having to take part in qualifying matches or be ranked at a particular level.
• a player or team granted a wild card.