"Blackjack" is a specific hand in the card game where a player gets an Ace and a 10-point card (10, Jack, Queen, or King) as their first two cards, and it’s the highest possible hand, typically paying 3:2. On the other hand, "21" refers to any hand that totals 21 points, which can be achieved with two or more cards. While all Blackjacks total 21, not all 21-point hands are considered Blackjacks.