Parthian and Arsacid are different names of the same ancient Iranian empire that ruled from 247 BC to 224 AD. The Parthian Kings, or Shahs, often used Arsaces as their crown names which is why some refer to this empire as the Arsacids. They are also called Parthian because they started by conquering the region of Partha in Iran's northeast and the adopted the language of that region, called Parthian Middle Persian.