The fictional world where most of J.R.R. Tolkien’s Middle-earth stories are set is called Arda.
In Tolkien’s mythology, Arda is the entire world created by the divine beings known as the Valar under the authority of Eru Ilúvatar. Middle-earth is only one major region of Arda, rather than a synonym for the whole world. The name is derived from Tolkien’s invented Elvish languages.
Arda changes dramatically across Tolkien’s imagined history. It includes the lands of the First Age, the island kingdom of Númenor, and the continents and seas known during the Third Age. Aman, the realm west of Middle-earth, is also part of Arda.
Readers sometimes confuse Arda with Valinor or Aman. Valinor is a region within Aman, while Aman is a continent or great land in the western part of the world. Tolkien’s stories therefore use several geographic levels, with Arda at the broadest scale.