The High Elven name for the whole world as it was conceived by Ilúvatar and shaped by the Valar. The word 'arda' means 'the realm', and it refers to Middle-earth as the kingdom (or realm) of Manwë. It included both the mortal lands of Middle-earth and the immortal realm of the Undying Lands.
Encyclopedia entry originally written by High_King_Of_Gondor