The Aether is a higher dimension than the third dimension that we perceive. The first direct description of the Aether that was presented is a radio on Moon. This radio has dialogue from Dr. Edward Richtofen, in which he states that the Aether is "another dimension", and that the M.P.D. is a "stable connection", and has a "direct connection" to the Aether. Doctor Monty says this to describe the Aether, "This is it. The Aether. The Infinite. The reality beyond. Beyond your world. Beyond your perception". The Aether is also where souls go after death. The Aether is theorized to be passed through upon teleportation, as well as used to explain many other things like the Mystery Box and Pack-a-Punch.
In the case of the Aether, the term "dimension" has a vastly different meaning than the term "universe(s)". The concept of the Aether being another dimension was introduced far before the concept of multiverses was introduced. When it comes to higher and lower dimensions, it is regarding the 1st dimension (x-variable), 2nd dimension (x-variable and y-variable), and 3rd dimension (x-variable, y-variable, and z-variable). The Aether is a dimension higher than the 3rd dimension that humanity perceives. Most people in the fields of science and others well-versed in the concept of dimensions believe the added variable in the 4th dimension to be time. This means that time can be traversed in the 4th dimension just as easily as space. Because we know that the Aether is a higher dimension than the 3rd, that means it is either the 4th dimension, or an even higher dimension.
Through a vast amount of character dialogue, we learn of the connection between the Aether and souls. Upon death, an organism's soul departs the deceased body and arrives in the Aether. If the corpse of an organism is reanimated, the soul of that organism is pulled out of the Aether, back into the 3rd dimension, and then is trapped inside the reanimated corpse. Once the reanimated organism dies, the soul returns to the Aether. When a soul departs an organism upon death, the corpse releases a small amount of Aetherial energy which can be used to power certain devices.
There are two instances where something has been called "The Rift". The Rift in Buried is described by Dr. Ludvig Maxis as the place that is closest to the Aether, thus Aetherial energy is at its strongest there. The Rift in Shadows of Evil, contrary to popular belief, is not the underground area of the map. The Rift in Shadows of Evil is the wormholes/portals that you use to traverse the map.
Having control over the Aether's energies allows for manipulation of time and space on a whim. This allows for the creation of time-loops, time travel, displacement of objects and people (across time, space, and potentially universes), and much more. The Moon Pyramid Device (M.P.D.) gives its conduit/user limited control over the Aether's energies. In Black Ops II, Maxis and/or Richtofen manipulated the TranZit crew into activating three towers at specific locations (one of them being the Buried Rift) to channel and manipulate the Aether's energies for the gain of Maxis and/or Richtofen.
Teleportation (Not Confirmed/Theoretical
It is heavily hinted that the use of higher dimensions (namely the Aether) is a crucial part of making the Matter Transference Device(M.T.D./Teleporter) work properly. This was first hinted in Der Riese, where there was wall writing that stated "Return Through Aether". This seems likely to be referring to traveling through the Aether via the M.T.D. to the Mainframe. This is also one of the most likely explanations for how the M.T.D. is capable of time travel, because it would imply going through a dimension where time isn't linear.
Chalk Drawings/Perk-a-Colas (Not Confirmed/Theoretical):
The Dr. Monty Blueprint Scraps on the map The Giant, has an abundance of symbolism of the 4th dimension. This scrap shows Hypercubes/Tesseracts, and a perplexing diagram of a wormhole. It heavily implies the use of the fourth dimension to displace objects across dimensions, universes, space, and time.
- The first mention of the Aether was the wall writing on the map Der Riese which stated "Return Through Aether".
- To date, there has not been one instance where souls were used to power things. Upon death, the soul goes to the Aether unless captured by the Summoning Key.
- The term Aether originates from Greek Mythology, where it was the "upper air" that the gods would breathe. It was later used as an element (like fire, water, etc.) to explain things such as travel through a vacuum. It is still used today by some people combined with the idea of fourth-dimensionalism (you can find this on many not-so-trustworthy websites that also speak of the Illuminati, the hollow earth, etc.).