Blocker
A card in your hand that reduces the opponent's possible combinations
Definition
A blocker is a card in your hand that prevents the opponent from having certain combinations, statistically reducing the probability they hold the strongest hands. Blockers are an advanced tool in building bluffing and defending ranges. The usefulness of a blocker is understood through combo counts: AA has 6 combinations (A♠A♥, A♠A♦, A♠A♣, A♥A♦, A♥A♣, A♦A♣). If you hold an ace, the opponent can only have 3 combos of AA instead of 6 — you cut the probability they have AA by 50%. That's why suited aces are preferred bluffs in 4-bets: they block opponent AA and AK. Blockers are especially powerful on the river where precision is at its peak. Bluffing with a blocker to the opponent's flush, straight, or nuts cuts the probability they hold those strong hands and makes your bluff more profitable.
You bluff on the river with A♦5♣ on a board with three diamonds (but you don't have the flush, just the ace of diamonds). You block the opponent's nut flush (with A♦ in hand, the opponent can't have the ace-high flush) AND you block AA (they can only have 3 combos of AA). Your bluff is significantly more effective than with a hand that has no blockers.