"21" is a name used for several card games where the objective is to reach or get close to the value 21. The most famous of these is Blackjack (often called "21" in casual speech), where players compete against a dealer, trying to have a hand total higher than the dealer's without exceeding 21.
There are also simpler family or pub variants where players compare totals among themselves, drawing cards in turn to approach 21. Card values typically assign face cards as 10, number cards as their pip value, and aces as 1 or 11.
Strategy in 21-type games involves risk management (when to draw another card), estimating probabilities, and, in casino Blackjack, knowledge of specific rules like dealer hit/stand on soft 17, splitting, doubling, and insurance.