Secured Armor (Armor Mastery, Combat)Source Armor Master's Handbook pg. 17
You are adept at using your armor to cover your most vulnerable areas.
Prerequisites: Base attack bonus +11 or fighter level 8th, armor training class feature, proficiency with medium or heavy armor.
Benefit: When you are hit by a confirmed critical hit or a sneak attack, there is a 25% chance that the critical hit or sneak attack is negated and damage is instead rolled normally.
Special: This chance stacks with the light fortification and moderate fortification armor special abilities.Source Armor Master's Handbook pg. 21 (Amazon)
If you fail to negate a critical hit against you, you can spend 5 stamina points to cause the attack to deal half damage. (See page 17.)