The minimum percentage of your range that must be defended against a bet to prevent the opponent from profitably bluffing a 0% equity hand. Inversely related to alpha; MDF = 1 - alpha. MDF = 1/(s+1) where 's' is the percentage of the pot that is bet. For example, if villain bets half-pot, hero needs to defend 1/(0.5+1) = 66% of their range in order to remain unexploitable against a pure bluff.