The Aether is not a plane, but often included when disucssing them. One can easily see into and experience the Aether with study. The Aether connects all the planes to each other and the material world. The aether is the place of invisible energy where magic is harnessed and shaped.
Mages who study Ley Lines and nexus often know the most about Aether. It is believed the lines nad nexus themselves may be concentrated Aether energy.
Any being from any plane can enter the Aether, but they must ultimately return to their plane of origin.