A martingale is a mathematical concept used in probability theory and statistics. It describes a sequence of random variables where the expected value of the next variable, given all prior variables, is equal to the current variable. This property makes martingales useful in various fields, including finance and gambling, as they help model fair games and predict future outcomes based on current information. In gambling, a common example of a martingale strategy involves doubling your bet after each loss, aiming to recover previous losses when a win eventually occurs. However, this approach can be risky, as it requires a substantial bankroll and can lead to significant losses if a long losing streak occurs.