A variable. An unknown.
A card in poker that can act as any other card. A joker is always a wild card.
The wild card team in American Major League Baseball or the NFL is a fourth team selected for the playoffs.

There are three divisions in each league/conference: East, West, and Central. The first place team in each division is entered into the playoffs, along with the next best team out of all three divisions.

A similar system is also used for the semi-finals in Jeopardy! Tournaments.

There are five quarter-final matches, resulting in five semi-finalists. There are nine spots available though, so four wild cards are chosen for the three semi-final matches.

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