In Norse mythology, Fimbulvetr (or fimbulvinter), commonly rendered in English as Fimbulwinter, is the immediate prelude to the events of Ragnarök. It means "great winter". In Old English it is pronounced as Fifelwinter.
Fimbulwinter is the harsh winter that precedes the end of the world and puts an end to all life on Earth. Fimbulwinter is three successive winters, when snow comes in from all directions, without any intervening summer. Innumerable wars follow.
The event is described primarily in the Poetic Edda. In the poem Vafþrúðnismál, Odin poses the question to Vafþrúðnir as to who of mankind will survive the Fimbulwinter. Vafþrúðnir responds that Líf and Lífþrasir will survive and that they will live in the forest of Hoddmímis holt.
The mythology might be related to the extreme weather events of 535–536, which resulted in a notable drop in temperature across northern Europe. There have also been several popular ideas about whether the particular piece of mythology has a connection to the climate change that occurred in the Nordic countries at the end of the Nordic Bronze Age from about 650 BC. 
In Denmark, Norway, Sweden and other Nordic countries, the term fimbulvinter is still used to refer to an unusually cold and harsh winter. However in Sweden, another common word is "vargavinter" ("wolf winter").
- Ström, Folke: Nordisk Hedendom, Studentlitteratur, Lund 2005, ISBN 91-44-00551-2 (first published 1961) among others, refer to the climate change theory.
- Svenska Akademiens Ordbok, entry for Fimbulvinter 
- Siv Strömquist (21 February 2011). "Gråbenstankar i vargavintern" (in Swedish). Svenska Dagbladet. Retrieved 6 October 2020.
- Gunn, Joel (2000). The Years Without Summer: Tracing A.D. 536 and its Aftermath (British Archaeological Reports International. Oxford, England: Archaeopress) ISBN 1-84171-074-1
- Keys, David Patrick (2000). Catastrophe: An Investigation into the Origins of the Modern World. (New York: Ballantine Pub) ISBN 0-345-40876-4.
- Larrington, Carolyne (Trans.) (1999). The Poetic Edda (Oxford World's Classics) ISBN 0-19-283946-2
- Lindow, John (2001). Norse Mythology: A Guide to the Gods, Heroes, Rituals, and Beliefs (Oxford University Press) ISBN 0-19-515382-0
- Orchard, Andy (1997). Dictionary of Norse Myth and Legend (Cassell) ISBN 0-304-34520-2