词典释义：
释义
n.
a playing card that can have any value, suit, colour, or other property in a game at the discretion of the player holding it
（由持牌人自由决定牌值、花色等的）百搭牌，变牌。
a person or thing whose influence is unpredictable or whose qualities are uncertain
未知（或不可预料）的人（或事），未知数。
【计 算机】 通配符。
an opportunity to enter a sports competition without having to take part in qualifying matches or be ranked at a particular level
外卡（指无需参加资格赛或排名赛即可参加体育比赛的机会）。
a player or team given such an opportunity
持外卡参赛的选手（或运动队）。
