The number 21 holds great significance in blackjack, as it represents the highest possible hand that players can achieve without busting. A hand totaling 21, particularly when composed of an Ace and a ten-value card (10, Jack, Queen, or King), is known as a "blackjack" and typically earns the player a higher payout than a regular win. This chance of hitting 21 is a key element of the game's strategy, influencing players' decisions on whether to hit, stand, or double down.