The minimum frequency your opponent needs to fold to a bet, in order to break even with a pure (0% equity) bluff. Alpha can be defined as risk/(risk + reward), where risk = amount bet or raised to win the pot, and reward = amount won when the opponent(s) fold(s). For example, if hero makes a half-pot bet, they are risking 0.5 pot to win 1 pot, so α = 0.5/(0.5 + 1) = 33%. Therefore, if the opponent folds more than ⅓ of the time, then hero has a profitable pure bluff with any two cards. Alpha (α) is inversely related to Minimum Defense Frequency (MDF), such that α = 1 - MDF