Bonus Armor refers to extra armor on any cloth, leather, mail, or plate items, or any armor that is on any other type of item, i.e. trinkets, rings, and necklaces.

Patch Changes

Patch 3.0.8 (2009-01-20): Bonus armor beyond the base armor of an item will no longer be multiplied by any talents or by the bonuses of Bear Form, Dire Bear Form, or Frost Presence.