In card games, card counting is the process of remembering which cards have been previously dealt and so gain an advantage in future hands by having an idea of the cards still to come. It is common not to try to remember precisely which cards have been seen, but to assign some sort of heuristic points score to each card in the deck(s) used and keep track of the overall score. This is particularly true of games played with many decks of cards at the same time.
Card counting is a crucial aspect of many card games - in particular bridge and blackjack. See Blackjack-Card Counting and Bridge-Techniques for full details of how card counting works in each game.
In certain poker games, keeping track of precisely which cards have been dealt is essential, for example, 7-card stud. This particular skill is not usually called "card counting", however.
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