Usually used to refer to a player holding a card that represents one of an opponent’s “outs.” For example, in hold’em if Player A has {10-Clubs}{10-Diamonds} and the flop comes {q-Spades}{j-Clubs}{9-Hearts}, the likelihood that Player B has a straight is reduced because Player A has two “blockers” (i.e., two of the four tens in the deck).

The concept often applies in Omaha because of the many draws that can arise in that game. Blockers are also frequently considered in lowball games like deuce-to-seven triple draw; e.g., a player dealt two deuces (very valuable cards in 2-7) has a couple of “blockers” to his opponents getting deuces.