The Old English word that inspired J.R.R. Tolkien’s name Middle-earth was middangeard.
Middangeard appears in Old English texts and refers to the inhabited world, or the human world situated between different cosmic realms. Tolkien adapted the concept and the modern English form “Middle-earth” for the setting of much of his mythology.
The term is not intended to mean a planet located physically inside Earth. It is related to older Germanic ideas about the world of human beings, including the Old Norse term Miðgarðr. Tolkien’s use of the name therefore connects his invented history with real medieval language and mythology.
Middle-earth is also not the name of the entire universe in Tolkien’s legendarium. His larger world is called Arda, while Middle-earth is a major region within it, especially the setting of The Hobbit and The Lord of the Rings. This distinction is often blurred in casual conversation, but it matters in Tolkien’s own fictional geography.