You are a master of using medium armor to avoid damage.
Prerequisite: Con 13, Light Armor Proficiency, Medium Armor Proficiency.
Benefit: Whenever you are wearing medium armor, you gain DR 1/–. This stacks with the DR gained from the Armor Mastery fighter ability and any DR gained from the material your armor is made from, but not with DR from any other source.
Adventurer’s Handbook: Genius Guide Volume 1