1. [n]  a playing card whose value can be determined by the person who holds it
2. [n]  an unpredictable factor; "the weather was a wild card"

Wildcard
While investigating the violent death of a traveler on a flight from Africa to London, Steven Dunbar, a member of the Sci-Med Directorate, makes a terrifying discovery.

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(From card games in which certain cards, often the joker, can act as any other card) A special character or character sequence which matches any character in a string comparison, like ellipsis ("...") in ordinary written text.

In Unix filenames '?' matches any single character and '*' matches any zero or more characters. In regular expressions, '.' matches any one character and "[...]" matches any one of the enclosed characters.

See also Backus-Naur Form.