The Ether (a.k.a. the ethereal realm or aether) is a realm separate from the physical plane. It is the source of magic. Scholars have likened the Ether to the ocean, with Sanctuary being the sky. By the same analogy, spells are the turbulence that occurs when the two interact with each other. However, much energy is lost in the transition between the physical and ethereal realms, meaning that only the strongest of humans can harness anything more than the most basic of magical effects, and even then, much energy is lost in the transition between the physical and ethereal realms. Electromagnatism exists on the cusp of both realms.
It is through the Ether that teleportation is made possible, traversing the Ether to instantly appear in a separate location. Manipulation of the Ether within the physical plane also allows the use of telekinesis, Assassins are also capable of using the Ether to charge their weapons.