Arda is the fictional world containing Middle-earth in J.R.R. Tolkien’s legendarium.
Tolkien used Arda as the name for the world created by Eru Ilúvatar and shaped by the Valar. Middle-earth is not the entire world; it is one major region of Arda, especially important in the histories told in The Hobbit and The Lord of the Rings.
The geography and condition of Arda change dramatically across the legendarium. It begins as a world shaped by the Valar, later includes the island kingdom of Númenor, and is transformed through events such as the destruction of Númenor and the changing of the world from a flat form to a round one in Tolkien’s mythology.
Aman and Valinor are often mistaken for names of the whole world. Aman is a continent west of Middle-earth, while Valinor is the land within Aman where many of the Valar dwell. Beleriand, meanwhile, is an earlier western region of Middle-earth that was destroyed at the end of the First Age.