In Norse Mythology, Yggdrasil was the "World Tree," a gigantic ash, thought to hold all of the different worlds: such as Asgard, Midgard, Utgard and Hel. The name literally means the 'horse of Yggr', i.e. the horse of Odin, since Yggr (meaning Dreadful) is one of Odin's many names.
Three roots supported the trunk, with one passing through Asgard, one through Midgard and one through Hel. Beneath the Asgard root lay the sacred Well of Urd, and beneath the Midgard root lay the spring or well of Mimir.
The messenger in the tree (and thus between the worlds) was the squirrel, Ratatosk. In the top of the tree was perched a giant eagle (with a hawk upon its forehead) who blew the winds over the worlds with his mighty wings. The roots of Yggdrasil were gnawed at by a dragon, Nidhogg. Heidrun, a goat, lived on top of Yggdrasil and ate its leaves. Four stags fed on the bark of Yggdrasil, including Dvalin.
Yggdrasil Linux was the name of an early Linux distribution.