wild card
1. In a card game, a card that the player can choose to represent any card. He's going to play his wild card as an ace to win.
2. Someone or something that is unpredictable. He is too much of a wild card to be a successful candidate. Nobody wants to vote for someone wildly unpredictable. The wild card in this game will be the weather.
3. A person or team who is permitted to participate in an event or competition, such as a round of playoffs, without having to adhere to the usual entry protocol. The wild card rarely wins the championship, but anything is possible.
4. In computing, a character that is substituted for an unknown character in a search. An asterisk is often used as a wild card in database searches.