AonDor is the magic system that we mainly see used in Elantris. It seems to only be able to be used by Elantrians. Shu-Dereth is able to make someone who is immune to the effects of AonDor.
AonDor involves the use of symbols called Aons, which can be drawn in the air in lines of light by an Elantrian to create a magic effect. An Aon must be drawn perfectly or it will not create the intended magic, and may cause dangerous results. The effect is determined by the specific Aon drawn.
Each Aon is built around the basic symbol known as Aon Aon. The symbol is often duplicated and rotated in the creation of another Aon. Aons can be made more specific by adding appropriate modifying lines to the symbol. For example, Elantrians have been known to specify which organs to heal and in what way with a healing Aon instead of leaving the symbol unmodified, resulting in a weakened healing of ailments throughout the body.
During the Reod, the configuration of the basic Aon changed, rendering all all existing Aons ineffective. That included a grand Aon, formed by the walls of Elantris and surrounding cities. Since that was the Aon that constantly increased the power of AonDor used by Elantrians, as well as causing their bodies to glow with an inner light, its failure has caused their bodies to get stuck in mid-transformed state, creating a corpse-like appearance, as well as inability to die or regenerate. Other Elantris Aons, used for lighting and indoor plumbing, has also failed, rendering the city almost unlivable.
It was later discovered by Raoden that the change in landscape of Arelon (in particular, a chasm opened by an earthquake) has been the cause of change in the Aon structure, and that adding the line corresponding to the chasm fixed the Aons' function.
AonDor is a compound of the words 'Aon' and 'Dor'. The Dor is understood to be a dimension of chaotic energy in a highly pressurized state. When an Aon is drawn, the symbol acts as a doorway for some of the energy to escape into the less pressurized world of Sel. The Aon not only acts as a doorway, but it specifies how that chaotic energy can be channeled into specific forms, such as fire, light, healing power, etc.