When on the receiving end of a melee attack, this stat is checked against to see how much damage you take.
It is compared against the attacker's Attack stat.
Defense is first calculated in the form: Defense = (8 + (VIT/2) + level1)
See also: Defense Bonus
1 If level is less than, or equal to 50, it is added as is without adjustment.
If level is greater than 60 it is added as level+10 (Lvl 60 would add in 70, 63 would add in 73, etc.)
If level is between 50-60, ((level x2)-42) is added, making the equation, 8 + VIT/2 + ((level x2) -42)
For levels 51-59, corrections to the calculations are needed as these numbers present inconsistencies. According to this formula, level 59 would use 76 while a level 60 would only use 70. It is not believed that one would gain 8 defense from obtaining level 51, and then lose 6 defense from obtaining level 60.