خراسان بزرگ or خراسان کهن) (also written Khurasan) is a historical region of Greater Iranmentioned in sources from Sassanid and Islamic eras which "frequently" had a denotation wider than current three provinces of Khorasan in Iran. It also included parts of Afghanistan, Turkmenistan, Uzbekistan and Tajikstan
Khorasan in its proper sense comprised principally the cities of Balkh, Herat and Ghazni (now in Afghanistan), Nishapur and Tus (now in Iran),Merv (now in Turkmenistan), and Samarqand and Bukhara (now in Uzbekistan). However, the name has been used in the past to cover a larger region that encompassed most of Transoxiana and Soghdiana in the north, extended westward to the Caspian Sea, southward to include theSistan desert and eastward to the Hindu Kush mountains in Afghanistan. Arab geographers even spoke of its extending to the boundaries of ancient India, possibly as far as the Indus valley, in what is now Pakistan. Sources from the 14th to the 16th century report that Kandahar,Ghazni and Kabulistan in Afghanistan formed the eastern part of Khorasan, overlapping with Hindustan.