Middle-earth is the name commonly used for the setting of most of J.R.R. Tolkien’s major stories.
The term refers specifically to a large region of Tolkien’s imagined world, including locations such as the Shire, Gondor, Mordor, Rohan, and Mirkwood. The wider world is called Arda, while Valinor is a blessed realm in the West and Beleriand is an earlier region whose history is central to The Silmarillion.
Tolkien adapted “Middle-earth” from older Germanic and Old English expressions related to the inhabited world between cosmic extremes. It does not mean a planet separate from Earth in Tolkien’s mythology. This distinction explains why some readers use Arda and Middle-earth interchangeably even though they are not exactly the same term.