Mehmed II (Ottoman Turkish: محمد ثانى, romanized: Meḥmed-i sānī; Modern Turkish: II. Mehmet Turkish pronunciation: [ˈikindʒi mehmet]; 30 March 1432 – 3 May 1481), commonly known as Mehmed the Conqueror (Turkish: Fatih Sultan Mehmet), 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 (Qayser-i Rûm), based on the assertion that Constantinople had been the seat and capital of the Roman Empire. The claim was only recognized by the Eastern Orthodox Church.
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 the city 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.
Source: Wikipedia, "Mehmed the Conqueror", available under the CC-BY-SA License.