Summary: The Martingale Strategy states that when a trader experiences a loss, they should immediately double the size of the next bet. By repeatedly doubling the bet when they lose, the trader will theoretically even out with a winning trade at some point.
In probability theory, a martingale is a sequence of random variables (i.e., a stochastic process) for which, at a particular time, the conditional expectation of the next value in the sequence is equal to the present value, regardless of all prior values.