Uncontroversially: from the Greek εις την Πόλιν [is tim polin], “to the City”—The City being the informal name of Constantinople in Greek, to this day. There is at least one similar Turkish placename: İstanköy is the Turkish for Kos (εις την Κω /is tin ko/).
There are some uncertainties about why it ended up as İstanbul instead of İstinbul. In fact there’s a paper I stumbled on in academia.edu, quoting me to support a Tsakonian origin for the /a/. (Impossible, the Tsakonians wouldn’t have shown up next to Erdek until 1700, and we’re talking two villages. But it’s nice to be quoted.)