The name Khorāsān is Persian (from Middle Persian Xwarāsān, sp. xwlʾsʾn', meaning "where the sun arrives from" or "the Eastern Province").[9][10] The name was first given to the eastern province of Persia (Ancient Iran) during the Sasanian Empire[11] and was used from the late Middle Ages in distinction to neighbouring Transoxiana.[12][13][14] The Sassanian name Xwarāsān has in turn been argued to be a calque of the Bactrian name of the region, Miirosan (Bactrian spelling: μιιροσανο,[15] μιροσανο, earlier μιυροασανο), which had the same meaning 'sunrise, east' (corresponding to a hypothetical Proto-Iranian form *miθrāsāna;[16] see Mithra, Bactrian μιυρο [mihr],[17] for the relevant solar deity).
Khorasan was first established as an administrative division in the 6th century (approximately after 520) by the Sasanians, during the reign of Kavad I (r. 488–496, 498/9–531) or Khosrow I (r. 531–579),[18] and comprised the eastern and northeastern parts of the empire. The use of Bactrian Miirosan 'the east' as an administrative designation under Alkhan rulers in the same region is possibly the forerunner of the Sasanian administrative division of Khurasan,[19][20][21] occurring after their takeover of Hephthalite territories south of the Oxus. The transformation of the term and its identification with a larger region is thus a development of the late Sasanian and early Islamic periods. Early Islamic usage often regarded everywhere east of Jibal or what was subsequently termed Iraq Ajami (Persian Iraq), as being included in a vast and loosely defined region of Khorasan, which might even extend to the Indus Valley and the Pamir Mountains. The boundary between these two was the region surrounding the cities of Gurgan and Qumis. In particular, the Ghaznavids, Seljuqs and Timurids divided their empires into Iraqi and Khorasani regions. Khorasan is believed to have been bounded in the southwest by desert and the town of Tabas, known as "the Gate of Khorasan",[22]: 562 from which it extended eastward to the mountains of central Afghanistan.[4][5] Sources from the 10th century onwards refer to areas in the south of the Hindu Kush as the Khorasan Marches, forming a frontier region between Khorasan and Hindustan
Khorasan has had a great cultural importance among other regions in Greater Iran. The literary New Persian language developed in Khorasan and Transoxiana and gradually supplanted the Parthian language.[52] The New Persian literature arose and flourished in Khorasan and Transoxiana[53] where the early Iranian dynasties such as Tahirids, Samanids, Saffirids and Ghaznavids (a Turco-Persian dynasty) were based.[citation needed]
Until the devastating Mongol invasion of the 13th century, Khorasan remained the cultural capital of Persia.[54] It has produced scientists such as Avicenna, Al-Farabi, Al-Biruni, Omar Khayyam, Al-Khwarizmi, Abu Ma'shar al-Balkhi (known as Albumasar or Albuxar in the west), Alfraganus, Abu Wafa, Nasir al-Din al-Tusi, Sharaf al-Dīn al-Ṭūsī, and many others who are widely well known for their significant contributions in various domains such as mathematics, astronomy, medicine, physics, geography, and geology