In Norse mythology, what was the name of the serpent that encircled Midgard?

In Norse mythology, the serpent that encircled Midgard was Jörmungandr, also called the Midgard Serpent.

Jörmungandr was one of the three monstrous children of the god Loki and the giantess Angrboða. According to myth, Odin cast the serpent into the sea surrounding Midgard, the human world. There it grew so large that it could encircle the earth and grasp its own tail.

The serpent is a major enemy of Thor. In one famous story, Thor tries to lift it while disguised as a cat during a visit to the giant Útgarða-Loki. At Ragnarök, Jörmungandr emerges from the sea and fights Thor; Thor kills it but dies after breathing in its venom.

Jörmungandr is distinct from Níðhöggr, who gnaws at the roots of the world tree, and from Fenrir, the giant wolf who is also Loki’s child.