wildcard

or wild card

1. Games A playing card whose value can vary as determined by its holder.
2. Sports An athlete or team selected to compete in a tournament or playoff from among those who did not qualify by meeting the regular requirements.
3. Computer Science A symbol that stands for one or more unspecified characters, used especially in searching text and in selecting multiple files or directories.
4. Slang An unpredictable or unforeseeable factor: A surprise witness proved to be the wild card at the trial.