In the card game of poker, a bluff is a bet or raise made with a hand which is not thought to be the best hand. To bluff is to make such a bet. The objective of a bluff is to induce a fold by at least one opponent who holds a better hand. The size and frequency of a bluff determines its profitability to the bluffer. By extension, the phrase "calling somebody's bluff" is often used outside the context of poker to describe situations where one person demands that another proves a claim, or proves that they are not being deceptive.

Poker is a family of comparing card games in which players wager over which hand is best according to that specific game's rules. It is played worldwide, but in some places the rules may vary. While the earliest known form of the game was played with just 20 cards, today it is usually played with a standard deck, although in countries where short packs are common, it may be played with 32, 40 or 48 cards. Thus poker games vary in deck configuration, the number of cards in play, the number dealt face up or face down, and the number shared by all players, but all have rules that involve one or more rounds of betting.