Natural resistance is an ability possessed by Prophecies bosses, Eye of the North bosses, some Nightfall bosses and some Boss-like foes which reduces the duration of hexes and conditions on them by half.
Notes
While this ability is in general beneficial, certain skills that have secondary effects upon hex/condition termination can exploit the half-duration caused by this ability, turning it into a liability, such as Wastrel's Worry, Lightning Surge, or Shatterstone.