noun
1.
( in card games ) a card that has no value of its own and takes the value of any card that the player chooses
2.
( sport ) an opportunity for sb to play in a competition when they have not qualified in the usual way; a player who enters a competition in this way
3.
( computing ) a symbol that has no meaning of its own and can represent any letter or number
4.
a person or thing whose behaviour or effect is difficult to predict