Magic Evasion is a stat that controls the ability to Resist a magical spell, just like (Melee) Evasion controls the ability to evade physical attacks.
Unlike melee evasion, only a few pieces of armor contain the term "magic evasion". Such an attribute can be found on augmented armors. The term is however used in the description of the Red Mage weapon skill Death Blossom.
Adding 1 point of magic evasion on the target of the spell (monster or player) is equivalent to removing 1 point of magic accuracy on the caster.
Magic evasion can be specified to apply to only certain spells or certain type of spells. For instance Barsleep grants magic evasion to sleep or repose and barfire grants magic evasion to all fire based spells (burn, Flare, etc).
The elemental resistance table under the equipment screen gives the relative increase/decrease in magic evasion to all 8 base elements.
1 point of elemental resistance is equivalent to 1 point of magic evasion for the specified element. For instance Crimson Cuisses give 20 magic evasion to Fire spells.
Different spells have different magic accuracies (for instance tier I elemental magic is less accurate than tier IV), or equivalently, players and monsters have different magic evasions depending on what spell is used.
level difference affects magic evasion, but it is not quantified yet. Magic evasion is the sum of a "natural" evasion and level difference. Mobs of different levels have different evasions, even if they are of the same type, and level difference is added on top of that.
Formula for magic landing rate
If a caster(A) with MA Magic Accuracy for that spell casts on a target(B) that has ME Magic evasion to that spell, the Magic Hit Rate of the spell is calculated as follows :
Magic hit rate %
MA ≤ ME-90
p = 5
ME-90 ≤ MA ≤ ME
p = 50-0.5*(ME-MA)
ME ≤ MA ≤ ME+45
p = 50+(MA-ME)
ME+45 ≤ MA
p = 95
Note that this only applies to the landing rate of the spell unresisted (full damage elemental magic or full duration enfeebling spell). The partial resist rates are a simple function of this rate (see Magic Hit Rate).
Determining Magic Evasion Value
The value of the magic evasion of the target is easily determined by checking you current landing rate (manually). The meva to the spell Stone of level 77 (65 INT) Qiqirn Poulterer in Alzadaal Undersea Ruins has been determined to be comprised between 290 and 292 using this method. This means that cap accuracy will happen with ~340 magic accuracy. Using the same method, VT Heraldic Imp in Caedarva Mire have been determined to have a M.eva for the spell sleep of around 390.
example 1 : If my landing rate is 40%, the target's M.eva is 20 points above the value of my magic accuracy.
example 2 : (Useful for testing) If my landing rate is 47% with +/- 5% error, the target's M.eva is between M-2 and M+16 where M is my magic accuracy.
example 3 : I have 320 elemental skill, 120 INT and 30 magic accuracy from gear (Jupiter's staff), the monster I target has 110 INT, and my landing rate is 80%. My total Magic Accuracy is easily computed : MA=320+30+10=360. When applying the Magic Hit Rate formula you see that the magic evasion of the target for the spell considered is ME=330.
Monsters of higher level than the player have much higher magic accuracy than the player's magic evasion. As a result, without use of magic evasion gear or barspells, resists will be very rare (5%). In order to see a landing rate that is not 5% or 95% (those are the caps), the caster has to have a Magic Accuracy comprised between ME-90 and ME+45, where ME is the magic evasion of the target.
For example, lvl 76 Friar's Lantern in Halvung have an estimated magic accuracy of ~ 150 above the player's natural magic evasion, which means that even with 90 fire resist from gear/barfire, they will land 95% of their spells on a level 75 character. A player needs around 230 fire resist to resist 95% of their spells and 150 fire resist to resist half of them.
In this example, the mob's magic accuracy was estimated while being nuked a substantial number of times with 168 fire resist, which gave a 32% no-resist rate.