What is the difference between Parthian and Arsacid Empire?

Context: The Parthian Empire, also known as the Arsacid Empire was a major Iranian political and cultural power in ancient Iran from 247 BC to 224 AD. Its latter name comes from its founder, Arsaces I, who led the Parni tribe in conquering the region of Parthia in Iran's northeast, then a satrapy under Andragoras, who was rebelling against the Seleucid Empire. Mithridates I greatly expanded the empire by seizing Media and Mesopotamia from the Seleucids. At its height, the Parthian Empire stretched from the northern reaches of the Euphrates, in what is now central-eastern Turkey, to present-day Afghanistan and western Pakistan. The empire, located on the Silk Road trade route between the Roman Empire in the Mediterranean Basin and the Han dynasty of China, became a center of trade and commerce.

Answer: 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.