You are trained at using your combat skill for defense as well as offense.
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 Armor Class. This number may not exceed your base attack bonus. The changes to attack rolls and Armor 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 Armor Class.
Special: A fighter may select Combat Expertise as one of his fighter bonus feats.
Source: Player’s Handbook (version 3.5), page 92.