Midgard is a realm in Norse mythology. Pictured as placed somewhere in the middle of Yggdrasil, Midgard is surrounded by a world of water, or ocean, that is impassable. The ocean is inhabited by the great sea serpent Jörmungandr (Miðgarðsormr), who is so huge that he encircles the world entirely, grasping his own tail. The concept is similar to that of the Ouroboros.
In Norse mythology, Miðgarðr became applied to the wall around the world that the gods constructed from the eyebrows of the giant Ymir as a defence against the Jotuns who lived in Jotunheim, east of Manheimr, the "home of men", a word used to refer to the entire world.
The realm was said to have been formed from the flesh and blood of Ymir, his flesh constituting the land and his blood the oceans, and was connected to Asgard by the Bifröst, guarded by Heimdallr.
The Fyrby Runestone.
According to the Eddas, Midgard will be destroyed at Ragnarök, the battle at the end of the world. Jörmungandr will arise from the ocean, poisoning the land and sea with his venom and causing the sea to rear up and lash against the land. The final battle will take place on the plain of Vígríðr, following which Midgard and almost all life on it will be destroyed, with the earth sinking into the sea, only to rise again, fertile and green.
Although most surviving instances of the word refer to spiritual matters, it was also used in more mundane situations, as in the Viking Agerunestone poem from the inscription Sö 56 from Fyrby:
The name middangeard occurs half a dozen times in the Old English epic poem Beowulf, and is the same word as Midgard in Old Norse. The term is equivalent in meaning to the Greek term Oikoumene, as referring to the known and inhabited world.
The concept of Midgard occurs many times in Middle English. The association with earth (OE eorðe) in Middle English middellærd, middelerde is by popular etymology; the continuation of geard "enclosure" is yard. An early example of this transformation is from the Ormulum:
þatt ure Drihhtin wollde / ben borenn i þiss middellærd
that our Lord wanted / be born in this middle-earth.