J.R.R. Tolkien used the fictional name Middle-earth for the central inhabited region of his mythology, broadly corresponding to the lands of northwestern Europe in his imagined past.
The term is not the name of the entire planet. Tolkien’s world is called Arda, while Middle-earth is a major region within it. The setting includes places such as the Shire, Gondor, Mordor, Rhovanion, and much of the action of The Hobbit and The Lord of the Rings.
Tolkien drew the expression from older Germanic and English words, including Old English middangeard and Old Norse miðgarðr. It is therefore more than a modern fantasy invention. A common mistake is to use Middle-earth as a synonym for all of Tolkien’s cosmology, even though Aman and other regions lie outside it.