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, west of Mannheim, "the home of men," a word used to refer to the entire world (there is no direct relation to the German city of Mannheim, which is attested from the 8th century, named after an early settler called Manno).
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 Bifrost Bridge, guarded by Heimdall.
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.
The name middangeard occurs half a dozen times in the Anglo-Saxon 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.
The usage of "Middle-earth" as a name for a setting was popularized by Old English scholar J. R. R. Tolkien in his The Lord of the Rings and other fantasy works; he was originally inspired by the references to middangeard and Éarendel in the Old English poem Crist.
Old English Earendel appears in glosses as translating iubar "radiance, morning star".
In the Old English poem Crist I are the lines (104–108):
éala éarendel engla beorhtast
ofer middangeard monnum sended
and sodfasta sunnan leoma,
tohrt ofer tunglas þu tida gehvane
of sylfum þe symle inlihtes.
Hail Earendel, brightest of angels,
over middle-yard to men sent,
and true radiance of the Sun
bright above the stars, every season
thou of thyself ever illuminest.
The name is here taken to refer to John the Baptist, addressed as the morning star heralding the coming of Christ, the "Sol Invictus". Compare the Blickling Homilies (p. 163, I. 3) which state Nu seo Cristes gebyrd at his aeriste, se niwa eorendel Sanctus Johannes; and nu se leoma thaere sothan sunnan God selfa cuman wille, that is, "And now the birth of Christ (was) at his appearing, and the new eorendel (morning-star) was John the Baptist. And now the gleam of the true Sun, God himself, shall come."
J. R. R. Tolkien was inspired by references in the Crist poem, deriving both the character Eärendil, also associated with the morning star, and his use of Middle-earth from it (see Sauron Defeated p. 236f.). The Quenya phrase, "Aiya Eärendil, elenion ancalima!", literally "Hail Eärendil, brightest of stars!", bears a strong similarity to the line "Hail Earendel, brightest of angels" in Crist I, even so far as to use the same syntax as the Old English version.