In Norse mythology, what is Jörmungandr (also called Midgardsorm), the great arch-enemy of Thor?

In Norse mythology, Midgardsorm (Jörmungandr) is a sea serpent. The serpent grew so large that he was able to surround the earth and grasp his own tail. As a result, he received the name of the Midgard Serpent or World Serpent. When he releases his tail, Ragnarök will begin. Thor will eventually kill Jörmungandr but will fall dead after walking nine paces, having been poisoned by the serpent's deadly venom.