noun: (computing) A character that takes the place of any other character or string that is not known or specified.
noun: (also written wild card) An uncontrolled or unpredictable element.
noun: (also written wild card) An element, often deliberately concealed, which is withheld for contingency.
noun: (sports, card games) Alternative form of wild card. [(card games) A card that can be assigned any value or used to substitute for any needed card.]