strategy
Blocker
Also: blockers, card removal, combinatorics
A card in your hand that reduces the number of ways an opponent can hold a specific hand. Holding the ace of spades on a three-spade board 'blocks' the nut flush.
Example. With A♠5♠ on K♠9♠4♦2♣7♥ you missed, but you block the nut flush, making an all-in bluff more credible.
Blockers work through combinatorics: if you hold an ace, there are three aces left instead of four, so there are three combinations of pocket aces instead of six. Blockers are the reason solvers pick particular hands to bluff — a hand that blocks the opponent's strongest calling hands is a better bluff, and a hand that blocks their bluffs is a better call. The reverse effect, holding cards that reduce the hands you want the opponent to have, is called an unblocker.