In poker, a bad beat occurs when a hand, which was at one time a big favourite to win, loses. Typically the term is only applied in this way when the player holding the eventual winning hand misplayed it spectacularly.
Alternatively, the term is also applied when a particularly strong hand loses to an even stronger one. In some casinos there is a "bad beat jackpot" awarded whenever a player suffers a particular beat.
A typical example of the first type of bad beat, in No Limit Texas hold 'em:
