In Norse mythology, what is the name of the serpent that encircles Midgard?

The serpent that encircles Midgard is Jörmungandr, also called the Midgard Serpent or World Serpent.

Jörmungandr is the middle child of Loki and the jötunn Angrboða, alongside Fenrir and Hel. Fearing a prophecy about Loki’s children, Odin removes them from Asgard. He casts Jörmungandr into the ocean surrounding Midgard, the human world. The serpent grows so enormous that it coils around the Earth and bites its own tail.

Its circular form resembles the ouroboros, a serpent representing a closed cycle by biting its tail. Jörmungandr is Thor’s great enemy, and their conflict appears in several myths. In one story, Thor tries to lift the serpent when it is magically disguised as a cat. In another, Thor hooks it while fishing with the jötunn Hymir, but the line is cut before Thor can kill it.

At Ragnarök, Jörmungandr releases its tail and rises from the sea, flooding the land and spreading venom. Thor kills it with Mjölnir but walks only nine steps before dying from the serpent’s poison. Níðhöggr is a different serpent-like creature associated with the roots of Yggdrasil.