Your Defense 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.
Your Defense is equal to the following:

10 + Reflex modifier + Dexterity modifier + other modifiers

Note that armor limits your Dexterity bonus, so if you’re wearing armor, you might not be able to apply your whole Dexterity modifier to your Defense. Sometimes you can’t use your Dexterity bonus (if you have one). If you can’t react to a blow, you can’t use your Dexterity bonus to Defense. (If you don’t have a Dexterity bonus, nothing happens.)

Other Modifiers

Many other factors modify your Defense.

Flat-footed: A character that is flat-footed receives no Reflex modifier, Dexterity modifier, deflection, or dodge bonuses to Defense.

Deflection Bonus: Deflection effects ward off attacks and improve your Defense.

Dodge Bonuses: Some other Defense bonuses represent actively avoiding blows. These bonuses are called dodge bonuses. Any situation that denies you your Dexterity bonus also denies you dodge bonuses. (Wearing armor, however, does not limit these bonuses the way it limits a Dexterity bonus to Defense.) Unlike most sorts of bonuses, dodge bonuses stack with each other.

Touch Attacks: Some attacks just need contact with the target, rather than a solid hit. In these cases, the attacker makes a touch attack roll (either ranged or melee). When you are the target of a touch attack, your Defense doesn’t include your Reflex modifier or any deflection bonuses. All other modifiers, such as your size modifier and Dexterity modifier apply normally.

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