Sira is the ancient word for the force that underlies the world. It’s actually a word from the ancient Tabersi (the predecessors of the Bitrani) that was sound-shifted into Calenyen; the Bitrani say shira..
The word sira fell out of favor around 1400 R, or approximately 400 years after the Rin and Girey story; the Academy story takes place in 1750 R (after the discovery of the continent of Reiassan). People didn’t so much stop believing in sira as start feeling that their explanations for it were too credulous, too superstitious, and too rooted in the very-powerful Temples of the Three. They wanted to understand more.
Enter the concept of aether.
This is another Bitrani word. The Calenyena call it aatur, simply using the Bitrani word shifted for a sound they find comfortable. Some scholars call it iezhyetar (from Iezhet, air, and ietar, power), but the term has not gained a great deal of popularity; ancient Bitrani sounds as if it has more gravitas to their ears.
As the scholars in 1750 R understand the aether (whatever they call it), the aether is an underlying force of the universe. It exists wherever two pieces of the world rub together, and can be harvested from these areas with the careful application of mechanical apparati or the very, very careful application of straight concentration and mental fortitude.
Very few scholars approve of the second method, however, because it has a feel very much like that of magic, and everyone knows that magic is bunk.