The first known name of the city is Byzantium (Greek: Βυζάντιον, Byzántion), the name given to it at its foundation by Megarean colonists around 660 BCE. The name is thought to be derived from a personal name, Byzas. Ancient Greek tradition refers to a legendary king of that name as the leader of the Greek colonists. Modern scholars have also hypothesized that the name of Byzas was of local Thracian or Illyrian origin and hence predated the Megarean settlement.
After Constantine the Great made it the new eastern capital of the Roman Empire in 330 CE, the city became widely known as Constantinople, which, as the Latinized form of "Κωνσταντινούπολις" (Konstantinoúpolis), means the "City of Constantine". He also attempted to promote the name "Nova Roma" and its Greek version "Νέα Ῥώμη" Nea Romē (New Rome), but this did not enter widespread usage. Constantinople remained the most common name for the city in the West until the establishment of the Turkish Republic, which urged other countries to use Istanbul. Kostantiniyye (Ottoman Turkish: قسطنطينيه) and Be Makam-e Qonstantiniyyah al-Mahmiyyah (meaning "the Protected Location of Constantinople") and İstanbul were the names used alternatively by the Ottomans during their rule. The use of Constantinople to refer to the city during the Ottoman period (from the mid-15th century) is now considered politically incorrect, even if not historically inaccurate, by Turks.
By the 19th century, the city had acquired other names used by either foreigners or Turks. Europeans used Constantinople to refer to the whole of the city, but used the name Stamboul—as the Turks also did—to describe the walled peninsula between the Golden Horn and the Sea of Marmara. Pera (from the Greek word for "across") was used to describe the area between the Golden Horn and the Bosphorus, but Turks also used the name Beyoğlu (today the official name for one of the city's constituent districts).
The name İstanbul (Turkish pronunciation: [isˈtanbuɫ] (listen), colloquially [ɯsˈtambuɫ]) is commonly held to derive from the Medieval Greek phrase "εἰς τὴν Πόλιν" (pronounced [is tim ˈbolin]), which means "to the city" and is how Constantinople was referred to by the local Greeks. This reflected its status as the only major city in the vicinity. The importance of Constantinople in the Ottoman world was also reflected by its Ottoman name 'Der Saadet' meaning the 'gate to Prosperity' in Ottoman. An alternative view is that the name evolved directly from the name Constantinople, with the first and third syllables dropped. A Turkish folk etymology traces the name to Islam bol "plenty of Islam" because the city was called Islambol ("plenty of Islam") or Islambul ("find Islam") as the capital of the Islamic Ottoman Empire. It is first attested shortly after the conquest, and its invention was ascribed by some contemporary writers to Sultan Mehmed II himself. Some Ottoman sources of the 17th century, such as Evliya Çelebi, describe it as the common Turkish name of the time; between the late 17th and late 18th centuries, it was also in official use. The first use of the word "Islambol" on coinage was in 1703 (1115 AH) during the reign of Sultan Ahmed III.
In modern Turkish, the name is written as İstanbul, with a dotted İ, as the Turkish alphabet distinguishes between a dotted and dotless I. In English the stress is on the first or last syllable, but in Turkish it is on the second syllable (tan). A person from the city is an İstanbullu (plural: İstanbullular), although Istanbulite is used in English.