You are trained at using your combat skill for defence as well as offence.
Prerequisites: Int 13.
Benefit: When you use the attack action or the full attack action in melee, you can take a penalty of as much as -5 on your attack roll and add the same number (+5 or less) as a dodge bonus to your Armour Class. This number may not exceed your base attack bonus. The changes to attack rolls and Armour Class last until your next action.
Normal: A character without the Combat Expertise feat can fight defensively while using the attack or full attack action to take a -4 penalty on attack rolls and gain a +2 dodge bonus to Armour Class.
Special: A Fighter may select Combat Expertise as one of his fighter bonus feats.
Source: Player’s Handbook (Page 92)