Bifröst (Scand.). A bridge built by the gods to protect Asgard. On it “the third Sword-god, known as Heimdal or Riger”, stands night and day girded with his sword, for he is the watchman selected to protect Asgard, the abode of gods. Heimdal is the Scandinavian Cherubim with the flaming sword, “which turned every way to keep the way of the tree of life”.
Source: H.P.Blavatsky - The Theosophical Glossary