How Istanbul got its name

After the fall of the Byzantine Empire in the late 15th century, the name "Constantinople" fell into disuse, with no obvious replacement.  But over the centuries in which Constantinople was the urban hub of the area (and, as you'll see, even after its fall), people referred to it as "the city."   

The Greek phrase for "in the city" is "εις την πόλη" which is pronounced "is tin Poli."  Over time, this became  "Istanbul."