In the work of the British author J. RR Tolkien , Arda is the name of the imaginary world created by Eru Ilúvatar and in which will unfold the history of Middle-earth during its different ages.
"Arda" means "Kingdom" in Quenya. (We can note the similarity of this name with the German Erde , the Dutch or Afrikaans Aarde and Arabic أرض / 'arḍ , which means" Earth ".)
Arda is actually Earth, created by Great Music, also called Ainulindalë (which gives this name to the first part of the Silmarillion), composed by the Ainur on a theme of Eru. Its shape has changed since then, among other things because of the wars against Melkor.
Arda was flat at first and was encircled by the Outer Sea called Ekkaia. Besides, it contained only one continent where the Valar lived, but Melkor secretly established himself in the north, digging the fortress of Utumno. One of the first upheavals of Arda was Melkor's assault on the two Lamps, Illuin and Ormal, creating oceans and mountains. The Valar took refuge in Aman, behind the Pelóri, the highest mountains of Arda, west of the Great Sea, abandoning Middle-earth.