Mage Armor is an property found on armor which allows for meditation in armor pieces which are non-medable, such as Platemail, Chainmail, Ringmail, Studded and Bone Armor. A full suit of Mage Armor would allow meditation the same as standard medable equipment, providing all set pieces have the Mage Armor property. It also negates the armor penalty on the Stealth skill.
Crafted Mage Armor
The following items receive Mage Armor property, if you craft them exceptionally. These are:
Mage Armor Artifacts
There are several more
Converting other armor into Mage Armor
As of Publish 81, "Mage Armor has been removed from imbuing, reforging, and enhancing. Armor now can be converted to and from Mage Armor by paying a Mage guild master 250,000 gold. Applying mage armor is still subject to the armor not having more than four properties or not being an artifact. Mage Armor now has an imbuing weight of zero."