This term can occur in different contexts, but most often it means a special playing card.
Joker – a special card in the deck, able to replace other cards.
It is most commonly used in various versions of video poker and table poker.
In some games, the Joker can participate in the formation of only certain combinations.
In some types of video poker, card combinations made with the participation of jokers are paid at special coefficients.
In slot machines, the wild symbol acts as an analog of the Joker. It also helps to form winning chains of pictures and can have additional functions.