Istanbul was formerly known as Byzantion (Byzantium in Latin), and this is the origin of the term we use today. It is not known with any certainty where the name 'Byzantion' came from, and it is quite clear that the legends that arose at later periods did not reflect the truth.

It has been noted that 'Byzant', which is the root of the word 'Byzantion' greatly resembles many of the place names existing in Entail during the third century. Although it can be accepted that the 'ion' suffix is associated with the Phrygians, who arrived with the Aegean migrations, the 'nt' on the end of the root 'Byzant' can also be found in the ancient local languages of Entail. Throughout the Early Ages the name 'Byzantion', which forms the core of the former name of Istanbul, was used. After the city had been re-founded in 330 AD by Constantine I (and this was towards the end of the Roman, Empire), it was referred to as 'Deutera Rome', or the second Rome', and also as 'Nea Rome', which means 'New Rome'. Then the name of its founder was taken as the basis, and the name 'Konstantinoupolis' adopted, which was the source of all the western names for the city. The Germans refer to Istanbul as 'Konstantinopel', the French and the British as 'Constantinople' and the Italians as 'Constantinopoli'. Although the official name of the city has, ever since the establishment of the Republic, been 'Istanbul' and great sensitivity shown on this subject, Europe resists the adoption of the name 'Istanbul'. It is not known with any certainty where the name 'Istanbul' came from. According to an opinion that has existed for many years, the Byzantines did not refer to the city by its actual name, but, because of it size, simply as 'Polis' (the City), and when they wanted to say 'to the City', they said 'eist enpolin' (is-tin-polin), which was the origin of the name 'Istanbul'. Recent research has shown that the name 'Istanbul' was used if not during the Byzantine period, at least during the 11th century and that the Turks knew the city by this name. Istanbul has had other names at various times but none of them was used widely or for any great length of time. During the Turkish period the names 'Dersaadet' and 'Deraliye' were used (and these were adjectival more than anything else), and if official correspondence and on coins the Turkish transcription of 'Konstantinoupolis', 'Konstantiniye' was used, Although the use of the name 'Konstantiniye' was prohibited at one time during the Ottoman period by Sultan Mustafa III, its use continued, to be abandoned during the republican period.