Middle-earth is the fictional world where most of J.R.R. Tolkien's best-known stories are set.
Tolkien used Middle-earth as the setting for The Hobbit, The Lord of the Rings, and much of The Silmarillion. It is a region of Arda, the larger mythological world Tolkien developed through stories, histories, languages, and invented genealogies.
The name does not mean a planet separate from Earth in Tolkien's conception. It corresponds broadly to the inhabited lands of a remote, imagined period of Earth's history. This is why maps of Middle-earth include familiar physical ideas such as seas, mountains, rivers, and regions with changing political boundaries.
The term is often mistakenly used for the entire universe of Tolkien's mythology. More precisely, Arda is the world, while Middle-earth is its central continent or region in the narratives most readers know.