Armor Specialization

 
Prerequisite(s): BAB 12+, Medium or Heavy armor proficiency
 
Benefit: Choose one type of Medium or Heavy armor with which you are porficient. When wearing masterwork armor (including magic armor), of that type, you gain Damage Reduction 2/-. If you lose your Dexterity bonus, you lose the benefit of this feat. You can gain this feat multiple times, each time it applies to a different type of armor.
 
Source: Players Handbook II