Khorasan, also written as Khurasan, is a historical region lying in the northeast of Persia, covering also parts of Central Asia and Afghanistan.

The name Khorasan is Persian and means "where the sun arrives from."

First established as a political entity by the Sassanids in the third century CE, the borders of the region have varied considerably during its 1600-year history.

Initially the Khorasan province of Sassanid empire includes the cities of Nishapur, Herat, Merv, Faryab, Taloqan, Balkh, Bukhara, Badghis, Abiward, Gharjistan, Tus or Susia, Sarakhs and Gurgan.

Some believe that at certain times Khorasan covered a wider area, which included parts of Transoxiana, Soghdiana, Sistan, and extended to the boundaries of the Indian subcontinent.

It acquires its greatest extent under the Caliphs, for whom "Khorasan" is 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 is divided into four major sections or quarters (rub′), each section based on a single major city: Nishapur (now in northeastern Iran), Merv (now in southern Turkmenistan), Herat and Balkh (both now in Afghanistan).When the Arabs first arrive to the southern Hindu Kush to defeat the Zunbils, they recognize it as al-Hind (Sind), owing to the prevalence of Buddhists and Hindus (non-Zoroastrians) due to its cultural connection with Greater India.

Sources from the fourteenth to the sixteenth century report that areas in the south of the Hindu Kush mountain range (Zamindawar, Balochistan, and Kabulistan) formed a frontier between Khorasan and Hindustan.

In the Islamic period, Persian Iraq and Khorasan are the two important territories.

The boundary between these two is the region surrounding the cities of Gurgan and Damghan.

In particular, the Ghaznavids, Seljuqs, and Timurids divide their empires into Iraqi and Khorasani regions.

The adjective Greater is added these days to distinguish the historical region from the Khorasan Province of Iran, which roughly encompasses the western half of the historical Greater Khorasan