The name of the serpent defeated by Thor in Norse mythology is Jörmungandr, also known as the Midgard Serpent. This formidable creature encircles the Earth and is prophesied to engage in a cataclysmic battle with Thor during Ragnarök. Their conflict exemplifies the eternal struggle between chaos and order, as Thor seeks to protect the realms from Jörmungandr's destructive potential. Additionally, this serpentine figure symbolizes the deep connections between the sea and the world of gods in Norse belief, highlighting complex themes in their mythology.