From Longman Dictionary of Contemporary Englishwild cardˈwild ˌcard noun [countable] 1 DGCa playing card that can represent any other card2 PREDICTsomeone whose behaviour or effect on a situation is difficult to guess3 a player who is a wild card or who is given a wild card is chosen for a competition although they have not previously done well enough to take part4 technicalTD a symbol that can represent any letter in some computer instructions
Examples from the Corpus
wild card• At this supreme moment in his plans, in his life, fate had suddenly dealt Jack a wild card.• China remains a wild card in the negotiations.• But it was like everybody already had their connections, and I was like this wild card.• Inevitably I fell for him, thereby complicating my life horrendously and introducing an unexpected wild card into his.From Longman Business Dictionarywild cardˈwild card noun [countable]1something that has an unexpected effect on a situationwild card inA wild card in the company’s prospects is the disagreement that has broken out between the company and some of its 16,000 workers.2COMPUTING a sign that can represent any letter in some computer commands 