Sultan was the title taken by the rulers of the independent Muslim kingdoms that sprang up in the wake of the breakup of the Abbasid khilafat at Baghdad. After the death of the Prophet (Sm), a khilafat was established, which was actually the commonwealth of the Muslims. The khilafat was divided into two or more, after the fall of the Umayyad khilafat of Damascus in 750 AD. Later there was further division of the khilafat, but the Abbasid khilafat in Baghdad continued upto 1258 AD. Taking advantage of the weakness of the Abbasid khilafat, numbers of independent Muslim kingdoms were established in the far-flung areas, eg the Ghaznavids, the Ghorids etc. The head of these kingdoms took the title of Sultan; they held authority with or without the recognition of the Abbasid khalifahs. All pre-Mughal Muslim rulers of India took the title of sultan.
The authority of the sultan found legal sanction from the writings of the jurists, they said whatever is not prohibited by the khalifah is legal or admissible. The Sultanate established in Delhi or other parts of India including Bengal was therefore lawful in the eyes of the Muslim jurists. Though the sultan, as a sovereign, was powerful to rule the country according to his will, his power was limited by the existence of Shariah or Islamic law. [Abdul Karim]
Bibliography IH Quraishi, Administration of the Sultanate of Delhi, 5th revised edition, 1971.