Uncanny Dodge

Barbarian's Class Ability

Uncanny Dodge (Ex): At 2nd level, a barbarian retains his Dexterity bonus to AC (if any) even if he is caught flat-footed or struck by an invisible attacker. However, he still loses his Dexterity bonus to AC if immobilized. If a barbarian already has uncanny dodge from a different class, he automatically gains improved uncanny dodge (see below) instead.

Source: Player's Handbook