 During the Achaemenid (550–330 BCE) and Parthian Empires (150 BCE – 225 CE), this region had been known by the Old Persian name "Athura".Asōristān, Middle Persian "land of Assyria", was the capital province of the Sasanian Empire and was called "Dil-ī Ērānshahr", meaning "Heart of Iran".The city of Ctesiphon served as the capital of both the Parthian and Sasanian Empire, and was for some time the largest city in the world.