Mehmed II (Ottoman Turkish: , romanized: Memed-i sn; Turkish: II. Mehmed, pronounced [icindi mehmed]; 30 March 1432 3 May 1481), commonly known as Mehmed the Conqueror (Ottoman Turkish: , romanized: Eb'l-fet, lit.'the Father of Conquest'; Turkish: Fatih Sultan Mehmed), was an Ottoman sultan who ruled from August 1444 to September 1446, and then later from February 1451 to May 1481. In Mehmed II's first reign, he defeated the crusade led by John Hunyadi after the Hungarian incursions into his country broke the conditions of the truce Peace of Szeged. When Mehmed II ascended the throne again in 1451 he strengthened the Ottoman navy and made preparations to attack Constantinople. At the age of 21, he conquered Constantinople (modern-day Istanbul) and brought an end to the Byzantine Empire.

After the conquest Mehmed claimed the title Caesar of the Roman Empire (Ottoman Turkish: , romanized: Qayser-i Rm), based on the fact that Constantinople had been the seat and capital of the surviving Eastern Roman Empire since its consecration in 330 AD by Emperor Constantine I. The claim was only recognized by the Patriarchate of Constantinople. Nonetheless, Mehmed II viewed the Ottoman state as a continuation of the Roman Empire for the remainder of his life, seeing himself as "continuing" the Empire rather than "replacing" it.

Mehmed continued his conquests in Anatolia with its reunification and in Southeast Europe as far west as Bosnia. At home he made many political and social reforms, encouraged the arts and sciences, and by the end of his reign, his rebuilding program had changed Constantinople into a thriving imperial capital. He is considered a hero in modern-day Turkey and parts of the wider Muslim world. Among other things, Istanbul's Fatih district, Fatih Sultan Mehmet Bridge and Fatih Mosque are named after him.

Sultan (; Arabic: سلطان sulṭān, pronounced [sʊlˈtˤɑːn, solˈtˤɑːn]) is a position with several historical meanings. Originally, it was an Arabic abstract noun meaning "strength", "authority", "rulership", derived from the verbal noun سلطة sulṭah, meaning "authority" or "power". Later, it came to be used as the title of certain rulers who claimed almost full sovereignty in practical terms (i.e., the lack of dependence on any higher ruler), albeit without claiming the overall caliphate, or to refer to a powerful governor of a province within the caliphate. The adjective form of the word is "sultanic", and the dynasty ruled by a sultan are referred to as a sultanate (سلطنة salṭanah).

The term is distinct from king (ملك malik), despite both referring to a sovereign ruler. The use of "sultan" is restricted to Muslim countries, where the title carries religious significance, contrasting the more secular king, which is used in both Muslim and non-Muslim countries.

In recent years, "sultan" has been gradually replaced by "king" by contemporary hereditary rulers who wish to emphasize their secular authority under the rule of law. A notable example is Morocco, whose monarch changed his title from sultan to king in 1957.