Persia has long been used by the West to describe the nation of Iran, its people, or its ancient empire. It derives from the ancient Greek name for Iran, Persis. This in turn comes from a province in the south of Iran, called Fars in the modern Persian language and Pars in Middle Persian. Persis is the Hellenized form of Pars, based on which other European nations termed the area Persia. This province was the core of the original Persian Empire. Westerners referred to the state as Persia until March 21, 1935, when Reza Shah Pahlavi formally asked the international community to call the country by its native name, Iran, which means Land of the Aryans.

~ from Wikipedia