Expertise( Player's Handbook 3.0)
You are trained at using your combat skill for defense as well as offense
When you use the attack action or full attack action in melee, you can take a penalty of as much as -5 on your attack and add the same number (up to +5) to your Armor Class.
This number may not exceed your base attack bonus.
The changes to attack rolls and Armor Class last until your next action.
The bonus to your Armor Class is a dodge bonus.
A character not capable of the Expertise feat can fight defensively while using the attack or full attack action to take a -4 penalty on attacks and gain a +2 dodge bonus to Armor Class.