The Bosporus is the strait through Istanbul that connects the Black Sea with the Sea of Marmara.
This narrow waterway runs roughly north–south through northwestern Turkey and separates Istanbul’s European and Asian sides. It is about 31 kilometres long, although its width varies dramatically, narrowing to roughly 700 metres at its tightest points.
The Bosporus is the northern half of the Turkish Straits system. Ships traveling from the Black Sea reach the Aegean and Mediterranean by continuing across the Sea of Marmara and then through the Dardanelles. That route has made the strait strategically important for trade, naval power and access to ports in countries such as Russia, Ukraine, Georgia, Romania and Bulgaria.
A common mix-up is the Dardanelles: it connects the Sea of Marmara to the Aegean Sea, not the Black Sea. The name Bosporus comes from an ancient Greek term meaning “ox-ford,” linked to the myth of Io, who was transformed into a cow.