Wild card
WordNet
noun
(1) A playing card whose value can be determined by the person who holds it
(2) An unpredictable factor
"The weather was a wild card"
WiktionaryText
Noun
- In card games, a card that can be assigned any value or used to substitute for any needed card.
- A competitor or team that is allowed to compete in a tournament despite not reaching the normal standards to qualify for inclusion.
- Kye Sun-Hui won an Olympic gold medal in Judo, despite being a wild card.