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"

(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.