Magicka, basically. The base of all creation.
Of course it's a bit deeper than that. Why not just call Magicka Creatia then? Like Paws said, its raw mythic energy that went into Creation in general, from the Mundus to Oblivion. I'm sure someone will come and put my explanation to shame, but Ill take a crack at it anyway.
It originates from Aetherius, where the Aedra reside. Some, if not all, Towers harvest Creatia from Aetherius, thereby stabilizing Creation and the Wheel itself. A major controversy that will probably come up in the future is that the Ayleids created their own Wheel in the form of White-Gold Tower and the towers around it; a "weapon" like this could do something horrible; what that "horrible something" is is up to debate. It's easy to determine that it would probably be bad, if not catastrophic.