The fictional world containing Middle-earth in Tolkien’s legendarium is called Arda.
Arda is the name for the world created within Tolkien’s mythology. Middle-earth is not the entire planet: it is one major region or continent of Arda, especially central to the stories of The Hobbit and The Lord of the Rings. Other important regions include Aman, where Valinor lies, and the lost lands of Beleriand.
Tolkien’s cosmology changes across his writings. Arda begins as a world shaped by the Valar under the authority of Eru Ilúvatar, and its history includes major geographical transformations. The destruction of Númenor is one event that changes the world’s form and separates the later geography from the earlier mythic ages.
Readers sometimes use “Middle-earth” as a synonym for Tolkien’s whole universe. In precise terminology, however, Middle-earth is only part of Arda, while Arda belongs to the larger creation known as Eä.