Bifrost (pronounced roughly “BEEF-roast;” Old Norse Bifröst) is the rainbow bridge that connects Asgard, the world of the Aesir tribe of gods, with Midgard, the world of humanity. Bifrost is guarded by the ever-vigilant god Heimdall. During Ragnarok, the giants will breach Heimdall’s defenses and cross the bridge to storm Asgard and slay the gods.
The etymology of the word is uncertain. The original form of the name seems to be Bilröst, which suggests a meaning along the lines of “the fleetingly glimpsed rainbow.” If Bifröst is correct, however, the meaning would be something akin to “the shaking or trembling rainbow.” In either case, the word points to the ephemeral and fragile nature of the bridge – just like the nature of a physical rainbow.
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 The Poetic Edda. Grímnismál, stanza 44.
 The Poetic Edda. Fáfnismál, stanza 15.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 36.