What is the name of the serpent that encircles Midgard?

The serpent that encircles Midgard is Jörmungandr, the Midgard Serpent of Norse mythology.

Jörmungandr is one of the three monstrous children of Loki and the giantess Angrboða; its siblings are Fenrir and Hel. According to Snorri Sturluson’s Prose Edda, Odin cast the serpent into the ocean surrounding Midgard. It grew so enormous that it coiled around the world and gripped its own tail.

Jörmungandr is Thor’s great enemy. In one famous episode, Thor hooks the serpent while fishing with the giant Hymir, but Hymir cuts the line before Thor can kill it. At Ragnarök, Jörmungandr releases its tail, emerges from the sea, and fights Thor. Thor kills it with Mjölnir, but the serpent’s venom kills Thor after he takes nine steps. Níðhöggr is a different dragon-like serpent associated with the roots of Yggdrasil, while Fenrir is the giant wolf.