The name of the world in J.R.R. Tolkien’s mythology is Arda.
Arda is the Quenya name for the world created within Tolkien’s mythology. It includes Middle-earth, but the two terms are not exact synonyms. Middle-earth is one major region or continent of Arda, while Arda encompasses the wider world and its changing lands, seas, and realms.
Tolkien’s creation story describes Arda as being shaped by the Valar under the authority of Eru Ilúvatar. Its geography changes substantially across the mythology, including the destruction of Númenor and the reshaping of the world after the fall of the Two Lamps and later upheavals.
A frequent mistake is answering “Valinor.” Valinor is the land of the Valar in Aman, a continent west of Middle-earth. Beleriand is a region of ancient Middle-earth, and Aman is another continent. Arda is the broader world-name that contains these settings.