What does the name Istanbul appear to come from?

A Greek phrase - eis tin polin, meaning to the city - which is what people in the surrounding region said when they were going there. Constantinople was so dominant that it did not need naming; you simply said you were going to the city, and the phrase hardened into a name.

Both names were in use for centuries before 1930, when Turkey made Istanbul official and asked foreign postal services to stop delivering mail addressed to Constantinople. The Ottomans had used Istanbul, Konstantiniyye and several others simultaneously, so this was a standardisation rather than a renaming.

The city had a similar naming history at the other end. Constantine founded it in 330 on the existing town of Byzantium and called it New Rome; nobody used that, and it became Constantine's city instead. So it has been named after a small Greek colony, an emperor's vanity, and a directional phrase that means nothing at all - and the only one that stuck describes it as the destination rather than the place.