Blackjack (formerly black jack or vingt-un) is a casino banking game. It is the most widely played casino banking game in the world. It uses decks of 52 cards and descends from a global family of casino banking games known as "twenty-one". This family of card games also includes the European games vingt-et-un and pontoon, and the Russian game Ochko [ru]. The game is a comparing card game where players compete against the dealer, rather than each other.

Blackjack's immediate precursor was the English version of twenty-one called vingt-un, a game of unknown provenance. The first written reference is found in a book by the Spanish author Miguel de Cervantes. Cervantes was a gambler, and the protagonists of his "Rinconete y Cortadillo", from Novelas Ejemplares, are card cheats in Seville. They are proficient at cheating at veintiuno (Spanish for "twenty-one") and state that the object of the game is to reach 21 points without going over and that the ace values 1 or 11. The game is played with the Spanish baraja deck.

"Rinconete y Cortadillo" was written between 1601 and 1602, implying that veintiuno was played in Castile since the beginning of the 17th century or earlier. Later references to this game are found in France and Spain.

The first record of the game in France occurs in 1888 and in Britain during the 1770s and 1780s, but the first rules appeared in Britain in 1800 under the name of vingt-un. Twenty-one, still known then as vingt-un, appeared in the United States in the early 1800s. The first American rules were an 1825 reprint of the 1800 English rules. English vingt-un later developed into an American variant in its own right which was renamed "blackjack" around 1899.

According to popular myth, when vingt-un was introduced into the United States (in the early 1800s, during the First World War, or in the 1930s, depending on the source), gambling houses offered bonus payouts to stimulate players' interests. One such bonus was a ten-to-one payout if the player's hand consisted of the ace of spades and a black jack (either the jack of clubs or the jack of spades). This hand was called a "blackjack", and the name stuck even after the ten-to-one bonus was withdrawn.

French card historian Thierry Depaulis debunks this story, showing that prospectors during the Klondike Gold Rush (1896–99) gave the name "blackjack" to the game of American vingt-un, the bonus being the usual ace and any 10-point card. Since blackjack also refers to the mineral zincblende, which was often associated with gold or silver deposits, he suggests that the mineral name was transferred by prospectors to the top bonus hand. He could not find any historical evidence for a special bonus for having the combination of an ace and a black jack.

In September 1956, Roger Baldwin, Wilbert Cantey, Herbert Maisel, and James McDermott published a paper titled "The Optimum Strategy in Blackjack" in the Journal of the American Statistical Association, the first mathematically sound optimal blackjack strategy. This paper became the foundation of future efforts to beat blackjack. Ed Thorp used Baldwin's hand calculations to verify the basic strategy and later published (in 1963) Beat the Dealer.

The object of the game is to win money by creating card totals higher than those of the dealer's hand but not exceeding 21, or by stopping at a total in the hope that the dealer will bust. Number cards count as their number, the jack, queen, and king ("face cards" or "pictures") count as 10, and aces count as either 1 or 11 depending on whether or not counting it as 11 would cause a bust. If a player exceeds 21 points, they bust and automatically lose. A total of 21 on the starting two cards is called a "blackjack" or "natural," and is the strongest hand.