Rescension is a combination of the words "Rescind" and "Ascenscion". The kindest way to describe it is a process that returns a world to a nonphysical state in the Third Realm, an event often alluded to in today's modern day religion. It is, in a sense, absolute destruction as well. There are different ways Rescension could occur, but one most relevant to the Aeternalae are how they're able to induce it by transforming as many Ningen as possible.
A good analogy is cutting the string of a balloon that lets it soar into the sky.