WordNet (r) 3.0 (2006):

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

The Free On-line Dictionary of Computing (30 December 2018):

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)