J.R.R. Tolkien’s invented world containing Middle-earth is called Arda.
In Tolkien’s mythology, Arda is the world or realm that includes Middle-earth and the western land of Aman. It is part of the larger created universe called Eä. This distinction matters because the terms describe different scales: Eä is the universe, Arda is the world, and Middle-earth is a major region within Arda.
Arda’s history changes dramatically across Tolkien’s imagined ages. It includes the shaping of the world by the Valar, the wars of the First Age, the rise and fall of Númenor, and the struggles of the Second and Third Ages. The geography and even the world’s form develop through mythic events.
Aman is not a synonym for Arda. It is a western continent or realm that includes Valinor, the home of many of the Valar and Eldar. Middle-earth likewise names a region, not the entire fictional universe. Tolkien’s layered terminology reflects his interest in creating a coherent mythology with its own cosmology.