Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

wild card
     
         (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}.
     
        (1997-07-16)