A blackjack is a short, thick stick that is used as a weapon. [US, informal] Police confiscated guns, knives and blackjacks. regional note: in BRIT, use truncheon.
You and fellow players are dealt two cards each whilst the dealer is dealt one face up. If your first 2 cards add up to 21 (an Ace and a card valued 10), that's Blackjack! If they have any other total, decide whether you wish to 'draw' or 'stay'. You can continue to draw cards until you are happy with your hand.