Accuracy determines your character's ability to land physical attacks. It is the counterpart of Defense. It uses the same mechanics as saving throws to determine if an attack hits. When you use Accuracy to inflict temporary physical effects on an enemy, every point your opponent's relevant saving throw exceeds your Accuracy will reduce the duration of the effect by 5%.
The Accuracy formula is:
4 + (Dexterity - 10) * 1.00 + Luck * 0.40 + Accuracy bonuses from + Accuracy bonuses from equipment + Accuracy bonuses or penalties from effects
Raw Accuracy is rescaled to produce the final Accuracy value.
In addition to this, for each point of effective accuracy over the enemy's defense (up to 100 excess accuracy), you get a bonus depending on your weapon type. The different types of bonuses can be found at Items.