Dead Card
A dead card is a card that is not legally playable.
The simple explanation
A dead card is out of play, for example because it has been exposed. It cannot be used by any player.
A closer look
Dead cards matter because they change the odds. A player who thought they had nine outs may have only eight if one of those cards has been exposed and is dead. Floor staff decide how to handle exposed cards, and the rules differ by game.
A practical example
A spade is accidentally exposed before the flop. It is a dead card, and a player with a spade draw has one fewer out.
Full explanation
A dead card is a card that is not legally playable.
A dead card is a card that cannot be legally played. A card that has been exposed or that is otherwise known to be unavailable is dead.
Dead cards reduce the number of outs. If a card you need is dead, you are less likely to complete your hand.
Rules for exposed cards differ by game and house. In some, the card is simply replaced, and in others the hand is redealt. Burn cards are also dead because they are removed from play before the deal.