In Norse mythology, what is the three-winter period before Ragnarok called?

In Norse mythology, the three-winter period before Ragnarok is called Fimbulwinter.

Fimbulwinter, from Old Norse Fimbulvetr, is an exceptionally harsh winter that precedes the final battle. The myth describes three successive winters with no intervening summer, accompanied by bitter cold, social breakdown, and violence among humans.

The winter is one stage in a larger chain of disasters. The sun and moon are pursued by wolves, the earth shakes, the dead and monstrous enemies advance, and the gods meet their final opponents. Fimbulwinter therefore functions as a sign that the established cosmic order is collapsing.

The term is sometimes used in modern fiction for any catastrophic winter, but its mythological meaning is more specific. It does not name Ragnarok itself, and it is not the same as Niflheim, the cold realm associated with the dead and primordial ice.