Aether is an element created by Xa'an. It is also sometimes referred to as the Upper Airs, the Upper Aether, and the Great Up and Out.
The Aether Domain is a domain fairly unique to this world. It is a "5th Element;" it is a space-filling substance that exists beyond the mundane and mortal world of rocks and air. Like air, it is gas-like, but thinner, purer, and more rarified. Creatures native to air cannot usually breath aether without an aid; it is analogus to water in that regard. Additionally, birds native to air cannot fly in aether; it is too thin for their crude wings.
This element is home to aetherial plants and creatures.
Additionally, it is closely connected to Magic and Dreams.
Those who travers the heavens pray to the god of Aether to grant them safe passage through his home.