https://en.wikipedia.org/wiki/Greater_Khorasan

Khorasan, also written as Khurasan (Middle Persian: Khwarāsān, Persian: خراسان بزرگ, or خراسان کهن), is a historical region lying in the northeast of Persia. "In pre-Islamic and early Islamic times, the term "Khurassan" frequently had a much wider denotation, covering also parts of Central Asia and Afghanistan; early Islamic usage often regarded everywhere east of western Persia, so-called Djibal or what was subsequently termed 'Irak 'Adjami, as being included in a vast and ill-defined region of Khurasan, which might even extend to the Indus Valley and Sind." ("Khurasan", The Encyclopaedia of Islam, page 55., Brill, Retrieved 2010-10-22.)