Midgard

In Snorri's Gylfaginning the sons of Borr, Óðinn and his brothers Vili and Vé, killed the giant Ymir and built the world from his body: the earth from his flesh, the mountains from his bones, and from his blood the sea, which they laid in a ring around the land. When Gangleri asks how the earth was arranged, High answers that it is round, with the deep sea lying around it. Along that shore the gods gave land to the families of giants. Further in, they built a stronghold around the world against the hostility of the giants, made it from Ymir's eyebrows and called it Miðgarðr. The first people, Askr and Embla, were shaped from two trees the brothers found on the shore, and the race descended from them was given its home in Miðgarðr. Grímnismál 41 says it in verse: from Ymir's brows the kindly gods made Miðgarðr for the sons of men.
In the stories Miðgarðr behaves like a border. A giantess lives east of it in the forest Járnviðr, where she gives birth to giants in the shape of wolves. Thor goes 'out over Miðgarðr like a young lad' to reach the giant Hymir. The tale of the giant builder is set early, after the gods had established Miðgarðr and built Valhalla, and the builder's offer is a fortress that would hold against mountain giants and frost giants even if they came in over Miðgarðr. Völuspá 4 places it at the start of creation: the sons of Burr raised up the lands, 'they who shaped glorious Miðgarðr', and the sun shone on the stones and the ground grew green.
Around all of it lies Jörmungandr, the Miðgarðsormr. Snorri says Óðinn threw Loki's serpent child into the deep sea that lies around all lands, and it grew until it lay in the middle of the ocean around every land, biting its own tail. Hymiskviða 22 calls it the encircler of all lands from below. Its enemy is Thor. Skáldskaparmál lists 'defender of Ásgarðr, of Miðgarðr' among his kennings, and Völuspá 56 names him Miðgarðs véurr, usually translated 'Midgard's defender', though the exact sense of véurr is uncertain. At Ragnarök he kills the serpent, walks nine steps and falls dead from its poison, and Surtr flings fire over the earth. Afterwards, Snorri says, the earth rises again out of the sea, green and fair.
The word is much older than the Icelandic texts and was used far beyond Scandinavia. Wulfila's Gothic Bible, translated in the 4th century, uses it in Luke 2:1 (midjungard, nominative midjungards), where all the world is to be registered. The Northumbrian copy of Cædmon's Hymn in the Moore Bede, c. 737, says God afterwards adorned middungeard as land for men; later Old English poets spelled it middangeard. In the Old Saxon Heliand, Christ's birth comes 'into this middilgard' to help many, and the Old High German Muspilli says that on the last day 'the moon falls, mittilagart burns'. All four are Christian texts, and in each the word means the world people live in. The Viking Age Fyrby runestone in Södermanland (Sö 56) uses it the same way when it calls the brothers Hásteinn and Holmsteinn the most skilled men 'in Miðgarðr'. J. R. R. Tolkien, who taught Old English at Oxford, took his Middle-earth from middel-erde, the Middle English form of the same word.