In Norse mythology, Yggdrasil is the World Tree, a great ash tree located at the center of the universe and joining the nine worlds of Norse cosmology. The trunk of the tree may be thought of as forming a vertical axis around which these worlds are situated, with Ásgard, realm of the gods, at the top and the Hel, located in Niflheim, at the bottom. Midgard, or Mid-Earth the world of mortals, is located in the middle and surrounded by Jötunheim, land of giants, both of which are separated by the ocean. Yggdrasil is also sometimes known as Mimameid or Laerad.
Source: http://en.wikipedia.org/wiki/Yggdrasil