Barbarian :: d20srd.org"Improved Uncanny Dodge (Ex)
At 5th level and higher, a barbarian can no longer be flanked. This defense denies a rogue the ability to sneak attack the barbarian by flanking him, unless the attacker has at least four more rogue levels than the target has barbarian levels. If a character already has uncanny dodge from a second class, the character automatically gains improved uncanny dodge instead, and the levels from the classes that grant uncanny dodge stack to determine the minimum level a rogue must be to flank the character."
Essentially, any class with uncanny dodge counts as a "rogue level" for the purposes of improved uncanny dodge. They just poorly named it after the main class it affects, rather than giving it a unique name that'd be less confusing.
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