The Old English term that inspired J.R.R. Tolkien’s Middle-earth is middangeard.
Old English middangeard referred to the human world or the inhabited lands between heaven and the underworld. Tolkien adapted the concept and the English form “Middle-earth” for the central region of his mythology. The term is related to the Old Norse miðgarðr, better known in English through the form Midgard.
Middle-earth is not the name of Tolkien’s entire universe. It is a continent or region within Arda, the world created in his legendarium. Arda itself belongs to a larger cosmological structure that includes places such as Aman and the Void.
Readers sometimes assume that Middle-earth means a planet called Earth in a modern science-fiction sense. Tolkien’s usage is instead rooted in older Germanic and Christian vocabulary for the human realm. His philological interest in historical words helped him give familiar English terms deeper mythic and linguistic associations.