Æther (pronounced as ear-far and known by less posh people Ether) is one of the original elements. Due to it's age it is completely different to anything that exists today and is beyond the comprehension of all but the most magically talented of people.
Æther is said to originate from the beginning of the universe when it swam down from above but made the mistake of not keeping moving and it's lower levels solidified into Gaia. Gaia allowed most of the Æther to drain down to below her while the rest remained and transformed into Neptune.
However something prevented Æther from leaving the universe and so it became trapped here doomed to forever be used to further the whims of mortals.
Direct manipulation of Æther is the most powerful and flexible form of magic, described as beautiful by some human magicians and denied by others who are annoyed by not being able to master its brilliance. However, usage of Æther is hindered by its difficultness to comprehend forcing people to use spells which are easier to use and understand (essentially acting as a shim between the human mind and the manipulation of the Æther).
Souls are made out of Æther and can be channelled in a form of Æther manipulation even most humans can grasp already being gifted with the Æther. It is, however, limited to the amount of Æther the person was born with.