"Blackjack" refers to the specific hand where a player is dealt an Ace and a 10-point card (10, Jack, Queen, or King) as their first two cards, which is the best possible hand. On the other hand, "21" simply refers to any hand that totals 21 points, but it doesn't necessarily have to be a Blackjack. So, while all Blackjacks are 21, not all hands totaling 21 are considered Blackjacks.