Editing 4SRD:Basics (section)

==ARMOR CLASS==
Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It’s the attack roll result that an opponent needs to achieve to hit you. <br>
Your AC is equal to the following: 10 + armor bonus + shield bonus (+ Dexterity or Intelligence modifier in Light Armour*<br>

Sometimes you can’t use your Dexterity or Intelligence bonus (if you have one). If you can’t react to a blow, you can’t use your Dexterity bonus to AC. (If you don’t have a Dexterity or Intelligence bonus, nothing happens.*<br>

Other Modifiers: Many other factors modify your AC.<br>
Enhancement Bonuses: Enhancement effects make your armor better.<br>
Feat Bonus: Certain feats improve your AC.*<br>
Power Bonus: Certain feats, powers or items provide “power bonuses” to improve your AC.*<br>
Item Bonus: Certain feats, powers or items provide “item bonuses” to improve your AC.*<br>
Bonuses of the same type cannot stack with each other. For example, you may receive benefit from a feat bonus and an item bonus at the same time, but not from two feat bonuses.*<br>