Yggdrasill (Ĭg´drəsĬl, yōōg´–), in Norse mythology, the great tree of the world. Its branches and roots extended through all the universe—the heavens, the earth, and the underworld. At its top sat an eagle, at its bottom twined a serpent, and between them ran a squirrel breeding discord. It was prophesied that at the doom of the gods the tree would be destroyed. See Germanic religion.