Short answers, pulled from the story.
The word Hindustan emerged from a sound shift that occurred between 850 and 600 BCE. Proto-Iranian speakers changed the letter s into h during this period to transform the Sanskrit term Sindhu into hapta hindu in the Avesta.
The 10th century text Hudud al-Alam defined Hindustan with its western limit formed by the river Indus running south to the Great Sea. The eastern boundary reached Kamarupa, which is present day Assam.
During the period when the Delhi Sultanate ruled northern territories between 1206 and 1526, Hindustan referred specifically to today's northern India, Punjab, and lands of the Indus. Mughal rulers made a further distinction between Hindustani and Hindu in their sources where Hindustani commonly referred to Muslims living in Hindustan.
Indian leaders rejected this naming due to the implied meaning of land belonging to Hindus and insisted the new Dominion should be called India instead. The name Hindustan did not receive official sanction from the Constituent Assembly of India while Bharat was adopted as the official name instead.
The Hindustani language derives from Old Hindi spoken in Western Uttar Pradesh and Delhi areas. Literary standard forms include Modern Standard Hindi and Modern Standard Urdu using different scripts.