WILDCARD

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."

A character that substitutes for other characters in regular expressions and globbing

A character that takes the place of any other character or string that is not known or specified.

"If the character * is acting as a wildcard, then the pattern a*m matches each of the words amalgam, atom and alum."

(also written wild card) An uncontrolled or unpredictable element.

(also written wild card) An element, often deliberately concealed, which is withheld for contingency.