See G. H. Fairbanks and B. G. Misra, Spoken and Written Hindi (1966); A. Rai, A House Divided: The Origin and Development of Hindi-Hindavi (1985).
Hindustani (Hindi: हिन्दुस्तानी) is an adjectival form of Hindustan which originally meant people from the whole geographical region of Indian subcontinent, though latterly it is used mainly to describe a region in northern India, east and south of Yamuna river, between the Vindhya mountains and the Himalayas, where Hindustani language is spoken and is the origin of Hindustani classical music, tradition, culture, and tehzeeb (etiquette).
In the Persian language, the word Hindi, which is itself derived from Sindh, Sanskrit for the Indus River + -stān, (Sanskrit for place) often formerly rendered Hindoostan and the adjective Hindustani, relates to various aspects of the geographical areas east of the Indus, or people living in it - the Hindustanis. Thus during medieval times it may have referred to the Indian subcontinent, while in modern usage 'Hindustan' has come to mean the Republic of India.
The adjective Hindustani is a term applied to the syncretic Hindu culture of South Asia. Hindustani is sometimes also used as an ethnic term applied to the whole of South Asia. For example, a West Indian man with roots in South Asia might describe his ethnicity by saying he is Hindustani. In a more restricted sense, the Hindustani people are those who are native speakers of the Hindustani language, as opposed to the other languages of India.