In Norse mythology, Jörmungandr (Old Norse: Jǫrmungandr, see Etymology), also known as the Midgard Serpent or World Serpent (Old Norse: Miðgarðsormr, "worm of Midgard"), is an unfathomably large and monstrous sea serpent or worm who dwells in the world sea, encircling the Earth (Midgard) and biting his own tail, an example of an ouroboros. As a result of him surrounding Midgard, the beast is referred to as the World Serpent. Jörmungandr releasing his tail is one of the signs of the beginning of Ragnarök.
Jörmungandr is said to be the middle child of the god Loki and the jötunn Angrboða. According to the Prose Edda, Odin took Loki's three children by Angrboða – the wolf Fenrir, underworld ruler Hel, and the serpent Jörmungandr – and removed them from Asgard (the world of the Æsir). The serpent Jörmungandr was tossed into the great ocean that encircles Midgard. There the serpent grew so large that he was able to surround the Earth and grasp his own tail. The old Norse thunder god, Thor, has a lengthy feud with Jörmungandr and the serpent is regarded as his archenemy. During Ragnarök, Thor and Jörmungandr engage in a ferocious battle, culminating in both of their deaths.
Etymology
The name Jǫrmungandr is a poetic title and consists of the prefix jǫrmun- and the word gandr. The prefix "jǫrmun-" denotes something huge, vast, or superhuman, by some speculated to be a noun, jǫrmunr, meaning the "world" (the vastness). The suffix "gandr" can mean a variety of things in Old Norse, but mainly refers to elongated entities and or supernatural beings. Gandr can refer to, among other things: snake, fjord, river, staff, cane, mast, stem, branch, penis, bind, and the like (mainly in a "supernatural" or "living" sense). The term "Jörmungandr" therefore has several possible meanings in connection with its mythology, such as: "the world serpent", "the world river" (a synonym for the sea where he dwells), "the world staff or branch" (a connection to the world tree Yggdrasil), as well as "the world bind" (the serpent's coiling around the world, biting its own tail, symbolising the world's circle of life).