Shield bonus

Shield Bonus
A shield bonus is a specific class of bonus to Armor Class normally provided by shields. Two shield bonuses do not stack. Note that Two Weapon Defense is not treated as shield bonus in DDO.

Shield bash causes the player to reliquish this bonus thus reducing their Armor Class unless they have the Improved shield bash feat.

Sources of shield bonus:
 * Items:
 * Shields +1-9 (magical enhancements contribute to the shield bonus by +1-5)
 * Spells:
 * Shield +4

Other common bonuses to your armor class are armor bonus, dodge bonus, deflection bonus, dexterity bonus, natural armor bonus, and miscellaneous bonus.