In Norse mythology, Jörmungandr is the giant sea serpent that encircles Midgard and bites its own tail.
Also called the Midgard Serpent or World Serpent, Jörmungandr was cast into the ocean surrounding Midgard by Odin. The serpent grew so enormous that it could coil around the world and grasp its own tail, creating an image similar to the ouroboros. Its release is described as one of the signs announcing Ragnarök.
Jörmungandr is the middle child of the god Loki and the giantess Angrboða. Its siblings include Fenrir, the monstrous wolf, and Hel, ruler of the underworld. The serpent’s principal divine enemy is Thor. In one famous episode, Thor nearly pulls Jörmungandr from the sea while fishing, but the line is cut before Thor can kill it.
At Ragnarök, Thor and Jörmungandr fight again. Thor kills the serpent but then dies after taking nine steps, overcome by its venom. Fenrir is a separate creature, not another name for the World Serpent.