wildcard
n. countablen. a person or thing that is unpredictable and could change the result of a situation. In computing, it is a symbol like * that can represent any character.
n. an unpredictable factor or person whose influence on a situation is difficult to estimate. In computing, refers to a character that can be substituted for any other character or sequence of characters in a search.
The new player is a wildcard who could win the game for us.
The weather remains the ultimate wildcard for the outdoor festival, as a sudden storm could force a total cancellation.
In the final weeks of the campaign, the third-party candidate emerged as a wildcard, potentially siphoning enough votes from the frontrunner to swing the entire election.
From wild + card.
Often used metaphorically in sports and politics to describe unpredictable participants.