Resistance

Resistances determine whether specific types of damage as resisted, and how much. These values can be seen on the character window (default key: P).

Until Deployment 14, resistances were expressed as percentages. This has since changed to a 'diminishing returns' system:

The Spy ability Polarity Field can be used to alter the damage resistances of enemies. Many skills, such as Rage, Resistance, Hortimonculus and Base Wave can improve allied resistances, as can wearing Hazmat Armor. Resistance does NOT cover all types of damage reduction -- some, such as that provided by cover, range, poor beading, or abilities like Shield Extender and Smoke Screen, reduce damage by fixed proportions instead.

Crafting modules exist for weapons and armor that improve specific resistances, and for weapons that have a chance of briefly lowering a specific resistance on its target. These modules, if they match the damage type of the weapon they're on, can substantially improve the weapon's damage output, although they function better on high-damage, rapid-fire weapons (particularly polarity guns) than on weapons with a slow refire rate. Weapon debuff modules cannot break an immunity.