In J.R.R. Tolkien’s writings, the fictional world containing Middle-earth is called Arda.
Arda is the world created by Eru Ilúvatar within Tolkien’s mythology. Middle-earth is only one major region of it, especially the setting of The Hobbit and The Lord of the Rings. Other parts of Arda include Aman, the western land associated with Valinor, and Númenor, the island kingdom that existed during the Second Age.
Tolkien adapted the term “Middle-earth” from older Germanic and English expressions related to the inhabited world. In his legendarium, it is not the name of the entire planet. Arda is often treated as an equivalent of Earth within a remote mythic past, though its geography and history are fictionalized.
A common mistake is to use Middle-earth and Arda as exact synonyms. Middle-earth usually means the central continent or region where many principal stories occur, while Arda includes the whole world and its changing lands, seas, and realms.