Card counting - Wikipedia

Card counting is a casino card game strategy used primarily in the blackjack family of casino games to determine whether the next hand is likely to give a probable advantage to the player or to the dealer. Card counters are a class of advantage players, who attempt to reverse the inherent casino house edge by keeping a running tally of all high and low valued cards seen by the player. Card counting allows players to bet more when the count gives an advantage as well as minimize losses during an unfavorable count. Card counting also provides the ability to alter playing decisions based on the composition of the remaining cards.