Istanbul ( or ; ), historically also known as Constantinople and Byzantium, is the most populous city in Turkey and the country's economic, cultural, and historic center. Istanbul is a transcontinental city in Eurasia, straddling the Bosphorus strait between the Sea of Marmara and the Black Sea. Its commercial and historical center lies on the European side and about a third of its population lives on the Asian side. The city is the administrative center of the Istanbul Metropolitan Municipality (coterminous with Istanbul Province), both hosting a population of around 14 million residents. Istanbul is one of the world's most populous cities and ranks as world's 6th-largest city proper and the largest European city.

.istanbul and .ist are approved top level domains (TLD) for the Internet. It is a community-based sponsored top-level domain by Istanbul Metropolitan Municipality and subsidiary Medya A.S. According to the Medya A.S., .istanbul will improve awareness on Istanbul's historical heritage and help economic growth by allowing unlimited and open registration of the names.