The æther was originally set apart from the physical world, but it now flows through and around it. The spirits of the dead exist in the æther, but they can also be used and contacted by clairvoyant humans on Earth. The adjective given to things relating to the æther is ethereal (e.g., ethereal threshold, ethereal batteries). Warden describes how the Rephaim once lived purely on æther, but lost the ability after the Waning of the Veils.
The outer darkness is technically part of the æther, but is often considered a separate realm. The phrase is taken from the Gospel of Matthew. It is a distance reach of the æther, very close to the last light, where spirits can no longer be reached by voyants. Spirits that travel to the outer darkness may be able to return, but it is thought to take a great deal of energy to call them back.
Spirits can be sent to the outer darkness with the threnody, a series of words uttered after death. It is often considered a mercy, as the spirit will no longer be tormented, but it may also be performed against the spirit's will.
The last light is a gateway at the farthest reach of the æther, just beyond the outer darkness. It is generally agreed that it leads to a final death from which there is no return. As nobody has every come back from the last light, next to nothing is known about it.
The word comes from the Greek αἰθήρ, referring to the essence breathed by the gods. In Greek mythology, æther (αἰθήρ, meaning "clear sky") was what the gods breathed, like mortals breath air. Aristotle considered it to be the fifth element.