Blocker
A blocker is a card in your own hand that reduces the number of strong hands your opponent can hold, because you are holding one of the cards they would need.
Blockers work on the level of ranges, not single hands. Holding one ace does not tell you what your opponent has, but it does remove every combination of their range that contains that exact card, which shifts the odds of the hands they can still have.
The effect is real but modest, and it is easy to lean on too hard. A blocker is a reason to shade a close decision, not a reason to make a bet that would otherwise be bad.
an example
your cards
board
Three hearts are on the board and your opponent bets big on the river. You hold the ace of hearts and a black deuce, so you have no flush yourself, but the best possible flush is not in their hand either: you are holding the card it needs. That does not mean they are bluffing, and it does rule out the one hand they would most like to have.
This example is made up. The real one is in your own hand history: find blocker in your own hands and see how often it came up and what it cost you.
related terms
Nuts
The nuts is the best hand possible on a given board, the one no other two cards can beat.
Range
A range is the whole set of hands a player could hold in a given situation, rather than a guess at the one hand they act ...
GTO
Game theory optimal play, usually shortened to GTO, means playing a strategy so balanced that no counter strategy can be ...
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Knowing the word is not the same as seeing it.
All ten hands in order, with how often each one turns up.