J.R.R. Tolkien called the world containing Middle-earth Arda.
In Tolkien’s mythology, Arda is the realm or world that includes Middle-earth and the lands of Aman. It was created within Eä, the wider universe brought into being by the supreme creator, Eru Ilúvatar. This layered cosmology is developed most fully in The Silmarillion and related writings.
Aman is not a synonym for Arda. Aman is a continent or realm to the west, associated with the Valar and the Elves who remained there. Valinor is a region within Aman, while Eä refers to the created universe as a whole. These names are often confused because Tolkien’s mythology contains several nested geographical and cosmic levels.
The word ‘Middle-earth’ refers primarily to the inhabited lands at the centre of Tolkien’s narrative world, not to the entire planet in every context. Arda’s history changes through the shaping of the world and the destruction of Númenor.