Blackjack
Blackjack, "also known as 21, a number which defines the object of the game," is a Human recreational gambling game.
The rules are as follow: "10s and picture cards are worth 10, ace -- 1 or 11 -- the other cards face value. The dealer must hit 17 -- five cards under pays double." (TNG: "The Royale")
On Theta VIII, blackjack was one of the games emulated in the physical represenation of the novel The Hotel Royale. When the USS Enterprise-D investigated the planet, Data proved to be a skilled Blackjack player. He beat the odds to get a "five card Charlie." (TNG: "The Royale")
Blackjack was also the code name of the ground controller for Captain John Christopher's mission to intercept the USS Enterprise (NCC-1701) during its inadvertent voyage to 1969. (TOS: "Tomorrow is Yesterday")
In the Bashir 62 holoprogram, Vic Fontaine, Frank Sinatra and Dean Martin lost fifteen thousand dollars in a blackjack game in 1958. (DS9: "His Way")
In early 2375, to avoid having to sing All the Way, Vic suggested to Worf that they should play "a little blackjack to get the blood pumping". (DS9: "Image in the Sand")
Kira Nerys and Frankie Eyes played blackjack later that year. (DS9: "Badda-Bing, Badda-Bang")