MDF
Minimum defence frequency: the smallest share of your range you must continue with against a bet so the bettor can't profit by bluffing any two cards.
MDF = pot / (pot + bet). It is a defensive floor, not a target: it tells you the most you can fold before a bettor could profit from bluffing their entire range, not how often you should actually call. Against a bet sized exactly the pot, MDF is 50%; a bigger bet lowers it, since folding more is fine when the bettor risks more to win the same pot, and a smaller bet raises it.