The main cities of Khorasan were Balkh and Herat (now in Afghanistan), Mashhad and Nishapur (now in northeastern Iran), Merv and Nisa (now in southern Turkmenistan), and Bukhara and Samarkand (now in Uzbekistan). The loosely defined region also included Transoxiana, Soghdiana, and Sistan and extended to the boundaries of the Indian subcontinent. Sources from the 14th to the 16th century report that areas in the south of the Hindu Kush mountain range (Zamindawar, Balochistan, and Kabulistan) formed a frontier between Khorasan and Hindustan.
Greater Khorasan is today sometimes used to distinguish the larger historical reason from the modern Khorasan Province of Iran (1906–2004), which roughly encompassed the western half of the historical Greater Khorasan.
First established as a political entity by the Sassanids, the borders of the region have varied considerably during its 1,600-year history. Initially the Khorasan province of Sassanid empire included the cities of Nishapur, Herat, Merv, Faryab, Taloqan, Balkh, Bukhara, Badghis, Abiward, Gharjistan, Tus or Susia, Sarakhs and Gurgan. In addition to these cities, Ibn Khordadbeh mentions the cities of Nasā, Marvrud, Zabulistan, Kabul, Termez, Bamyan, Sogdia, Farghana, Rivsharan, Jowzjan, Khwarazm, Khotl, Osrushana, Sajistan, Pushang, Kesh, Botam, Vardana, Gorgan and Transoxiana as part of Khurasan. So according to him, eastern parts of today Iran, the entire Afghanistan, Turkmenistan, Uzbekistan, Tajikistan, Kyrgyzstan, southern parts of Kazakhstan, northwestern regions of China (Xinjiang), and the Indus Valley were part of Khurasan.
It acquired its greatest extent under the Caliphs, for whom "Khorasan" was the name of one of the three political zones under their dominion (the other two being Eraq-e Arab "Arabic Iraq" and Eraq-e Ajam "Non-Arabic Iraq or Persian Iraq"). Under the Umayyad and Abbasid caliphates, Khorasan was divided into four major sections or quarters (rub′), each section based on a single major city: Nishapur, Merv, Herat and Balkh.
In the Middle Ages, the term was loosely applied in Persia to all its territories that lay east and north east of Dasht-e Kavir and therefore were subjected to change as the size of empire changed. According to Ghulam Mohammad Ghobar, Afghanistan's current Persian-speaking territories formed the major portion of Khorasan, as two of the four main capitals of Khorasan (Herat and Balkh) are now located in Afghanistan. Ghobar uses the terms "Proper Khorasan" and "Improper Khorasan" in his book to distinguish between the usage of Khorasan in its strict sense and its usage in a loose sense. According to him, Proper Khorasan contained regions lying between Balkh in the east, Merv in the north, Sistan in the south, Nishapur in the west and Herat, known as the Pearl of Khorasan, in the center. Improper Khorasan's boundaries extended to as far as Hazarajat and Kabul in the east, Sistan and Baluchistan in the south, Transoxiana and Khwarezm in the north, and Damghan and Gorgan in the west. It is mentioned in the Memoirs of Babur that: