What does the name Middle-earth derive from in Tolkien's mythology?

Tolkien’s name Middle-earth derives from the Old English term middangeard.

In Old English, middangeard referred to the inhabited world, the realm between heaven and the underworld. Tolkien adapted the concept for his legendarium, using Middle-earth as the setting for the stories of Elves, Men, Dwarves, Hobbits, and other peoples.

The term is related to the Old Norse miðgarðr, or Midgard, which appears in Norse mythology. Tolkien’s Middle-earth is therefore not simply another name for the entire planet Earth, nor is it a separate planet. It is a historical region or world-age within the larger cosmology of Arda.

In Tolkien’s internal chronology, the events of The Hobbit and The Lord of the Rings occur during the Third Age. The geography of Middle-earth changes dramatically after major events such as the sinking of Númenor and the end of the Second Age. This layered history explains why the term carries both linguistic and mythological significance.