SRD:Fortification

Fortification: This suit of armor or shield produces a magical force that protects vital areas of the wearer more effectively. When a critical hit or sneak attack is scored on the wearer, there is a chance that the critical hit or sneak attack is negated and damage is instead rolled normally.

Strong abjuration; CL 13th; Craft Magic Arms and Armor, limited wish or miracle; Price varies (see above).