Dictionary.net

In Scand. myth. the ash-tree which binds together heaven, earth, and bell. Its branches spread over the whole world and reach above the heavens. Its roots run in three directions: one to the Asa gods in heaven, one to the Frost-giants, and the third to the under-world. Under each root is a fountain of wonderful virtues. In the tree, which drops honey, sit an eagle, a squirrel, and four stags. At the root lies the serpent Nithhoggr gnawing it, while the squirrel Ratatoskr runs up and down to sow strife between the eagle at the top and the serpent at the root.