At the end of the game when Ciri stops the white frost what exactly is it?

I haven't read the books but from playing through all the games I'd assumed it was simply climate change however this doesn't seem to be the case.

From what I understood, it's their version of the apocalypse. When the world will basically freeze over because of the "invasion" of the wild hunt. Something like that.

It is the Apocalypse of the Witcher series. The eventual destruction of the unnamed continent and islands where all the kingdoms and realms exist.

It is an Elven prophesy by Ithlinne.

The original Witcher showed this in some degree where you meet the "promised one" who will lead man to his salvation.

Is it actually tied to the Wild Hunt, though? I thought they had just figured out how to use it as a weapon.

I think that's why it's regarded as the apocalypse. The Wild Hunt are signifiers of war and destruction, and thus it would make sense that they would cause the end of the world. Plus, I think one of the comics talks about how the plan of the Wild Hunt is to take over this world. They control the power of the white frost, hence all the ice and stuff, so I think they go hand in hand.

There's an in-game book that mentions it, theorizes it as, basically, a super-cold spatial anomaly; freezes whatever world it runs into; which one could then infer that the Wild Hunt's world has hit it; given their existence and agenda, it's possible, but not easy, to live in the world after the fact, which is why they want to invade the witcher's, I guess?

Actually, they are scared of it! The White Frost was already consuming their world, that's why they tried to get Ciri: they needed her in order to do something about it. What? I don't know, maybe moving their people to another world and conquer it (since their ability to teleport people from one world to another was limited), maybe try to stop the White Frost... When you go to Tir Na Lia with Avallac'h you can find some kind of report, where it is stated that the elven mages were trying to stop the White Frost in their world, with no success.