Fortitude

Your constituation score increases by 1, to a maximum of 20

Accordingly you choose 1 type of physical damage type (slashing/buldeoning/piercing) as well as 1 elemental damage type from: acid, cold, fire, lightning, poison or thunder. Whenever you are harmed by the selected type of damage you can decrease dealt total by an amount equal to your constitution modifier to a minimum of 1 damage.

This takes places before any other form of damage reduction.