Shield bonus

A shield bonus is a specific class of bonus to Armor Class normally provided by shields. Multiple shield bonuses do not stack.

Sources of shield bonuses other than actual shields can be useful to characters who don't use shields (such as bow-users, two-handed-weapon wielders, wearers of rune arms or orbs, and spellcasters wishing to avoid Arcane Spell Failure.)

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: base +1 to +12 plus additional enhancement bonus +1 to +7 or more
 * Non-shield named items that grant shield bonus to AC
 * Spells:
 * Shield +4
 * Class abilities:
 * Ranger Tempest enhancements, specifically Shield of Whirling Steel and Improved Defense, both when dual-wielding.

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