(also spelled Yggdrasill), in Norse mythology, an ash tree, also called the World Tree. Yggdrasil apparently means "the horse of Yggr," Yggr (Terrible One) being one of the names of the god Odin. This immense, nurturing tree was the central feature and one of the most original creations of Norse cosmology. It supported all the nine realms of the Norse universe, branching out over the entire world and up into heaven. The 'Prose (or Younger) Edda' describes Yggdrasil as the holy place of the gods, where they held court each day as silver drops of dew trickled over the trees leaves. Poles and pillars are elements in myths of diverse cultures, but Yggdrasil's importance to the Norse worldview may reflect the Germanic belief in the sacredness, and consequent worship, of particular trees.

Yggdrasil as a yew tree, with yew berries to represent the Nine Worlds.

Many mythic animals were associated with Yggdrasil. A wise eagle perched in the topmost branches, surveying the whole world. A hawk, Vedurfolnir, sat perched between the eagle's eyes. Four stags, named Dain, Dvalin, Duneyr, and Durathror, continually devoured the tree's leaves and nibbled at its bark, while the goat Heidrun, whose mead-milk was drunk by Odin's warriors in Valhalla, also fed on its leaves. Many creatures, especially a cruel and frightening dragon named Nidhogg (or Nidhoggr, meaning "dreaded biter"), ate away at Yggdrasil's roots from below. A squirrel named Ratatosk (Travel Tusk) skittered up and down the tree trunk delivering spiteful insults between the eagle at the top and Nidhogg at the bottom. Bees fed on the honeydew that dripped from the tree, and the first two swans that ever existed swam in the water near the root that emerged in Asgard, the land of the gods.

Yggdrasil had three roots, each of which was associated with a body of water. The deepest root descended all the way to Niflheim, the realm of death. It was there that the dragon Nidhogg lay, in the dank well Hvergelmir (which means "roaring cauldron" or "bubbling cauldron"), the source of all the rivers in the world. Another root ended in Jotunheim, home of the frost giants. Beneath this root bubbled Mimisbrunn (Mimirs Well). It was the fountain of knowledge, prophecy, and poetry, and its guardian was Mimir, the personification of wisdom, who drank from the well every day, using his drinking horn, Gjallarhorn. The price of knowledge was steep, however. When the most powerful god, the war god Odin, came and asked Mimir for a single drink from the well, Odin had to give one of his eyes in payment. He accepted the price, and after he drank, he was suddenly so wise that he also became the god of seers, poets, and sorcerers.

Under the third root, the one that emerged in Asgard, the heavenly home of the gods, was the holy well Urdarbrunn (Urd's Well), also called the Well of Fate or Weird's Well. The water from this well was so sacred that, according to the 'Prose (or Younger) Edda', anything it touched turned as white as the inside of an eggshell. It was at this well that the gods held their council, riding to it each day over Bifrost, the bridge that linked Earth and heaven. Near the well was a great hall, home of the three Norns, or Fates--Urd, Verdande and Skuld, or Past, Present, and Future--who ruled over the destinies of mortals and gods alike.

Despite the constant loss of its leaves, the rotting of its trunk, and the gnawing away of its root by the great dragon Nidhogg, Yggdrasil always remained alive and green because every day the Norns drew water from the Urdarbrunn and sprinkled it over the tree. They also patched healing clay from the well onto the tree trunk in the places in which it had rotted and been eaten away by the stags; this kept the tree's limbs from withering.

Through its roots and branches Yggdrasil was connected with all the principal divisions of the world; it was also connected, symbolically, to the past through the wise Mimir and to the future through the Norns. It magically conferred knowledge as well: according to one myth, Odin hanged himself from Yggdrasil in order that the sacred runic secrets would be revealed to him.

According to tradition, the trembling of the World Tree would signal the beginning of the end of the world, Ragnarok, and the great final battle between the gods and the giants. At that time, the 'Prose (or Younger) Edda' says, Yggdrasil "will shake and nothing will then be unafraid in heaven or on Earth." Yggdrasil would remain standing after Ragnarok, according to one story, and two humans would emerge from the tree and repopulate the world.