Wildcard
Etymology
From card games, where a "wild card" can stand for any card the holder chooses. Computing borrowed it for a symbol matching any string, and sport for a competitor granted entry without qualifying. All three senses share the idea of an unfixed value that could turn into anything.
noun · /WYLD-kard/
An unpredictable element, or an entrant admitted outside the normal qualifying route.
What it names
The factor that can’t be modeled — the slot or person whose behavior is undetermined.
Example
The negotiations were going smoothly until his unpredictable deputy showed up as the wildcard.
Related idea
A wildcard is unpredictable by nature, whereas a dark horse is merely unknown, and could prove entirely ordinary once seen.
noun · /WYLD-kard/
A symbol, such as * or ?, that stands in for any character or characters in a search or file name; also, a playing card that can represent any other.
What it names
a stand-in for any value
Example
Type *.txt to match every text file.
Related idea
The unpredictable-person sense grows from the card that could be anything. Compare gambit and metagame.