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The D&D mechanical abstraction of how difficult it is to hit and cause damage to an opponent.

Armor Class (AC) is an abstraction of how hard it is to both hit and damage a foe in D&D and D&D-derived RPGs. It functions as a target number to meet or beat on an attack roll. Missing an attack roll may mean that the target was missed, or managed to shrug off or deflect the blow to no ill effect, depending on the interpretation of the abstraction.

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