Mage Armor is a passive level 56 Arcanemage ability that increases the mage's Mastery, and reduces the duration of all harmful Magic effects used against the mage by 25%. Mastery is increased by 3000 at level 90.
In PvP, the 25% debuff time reduction can be a life-saver, especially when facing warlocks and priests, since a great deal of their damage comes from damage over time effects and dispellable curses.
Patch 6.0.2 (2014-10-14): Now a passive effect and is available only to Arcane mages.
Patch 5.1.0 (2012-11-27): Now reduces the duration of harmful Magic effects by 25%, down from 35%.
Patch 5.0.4 (2012-08-28): Redesigned. Mage Armor now increases Mastery, and no longer regenerates mana or increases Resistances. This spell is now available at level 80, up from 68. In addition, a 3-second cast time has been added, there is no longer a duration, and it now persists through death.
Now reduces the duration of harmful Magic effects by 35%, down from 50%.
Patch 4.0.1 (2010-10-12): Redesigned. Now regenerates 3% of the mage's maximum mana per 5 seconds. In addition, Mage Armor is now trained at level 68, up from 34.
Patch 3.1.0 (2009-04-14): Now grants 50% of mana regeneration while casting, up from 30%.
Patch 3.0.2 (2008-10-14): Mage Armor will now reduce all harmful Magic effects by 50%, in addition to its current effects.
Patch 2.4.3 (2008-07-15): No longer considered a magic effect, and cannot be dispelled.
Patch 1.5.0 (2005-06-07): Now has a new, unique icon.
Patch 7.1.5 (2017-01-10): Removed. 
Patch 6.0.2 (2014-10-14): Now a passive effect and is available only to Arcane mages. 
