From DDO wiki
Armor Class Bonus (monk)
Monks get a stacking bonus to their AC:
- A passive Wisdom bonus equal to their Wisdom modifier, while unarmored and not using a shield.
- Centered bonus at levels 2, 5, 10, 15 and 20 (3, 6, 9, 12 and 15 AC) unarmored, wielding ki weapons and not encumbered.