The Bosporus or Bosphorus  is a strait (41°07 ’10 North, 29° 04′ 31 East) that separates the European part of  Turkiye from its Asian part , connecting the Sea of Marmara  with the Black Sea. It is an important oil transit chokepoint. It is 30 km long, with a maximum width of 3,700 meters at the northern entrance, and a minimum width of 750 meters between Anadoluhisarı and Rumelihisarı. The depth varies from 36 to 124 meters in midstream. It is a former river valley that was drowned by the sea at the end of the Tertiary period. The city of Istanbul straddles the Strait with a population of more than 11 million people.

Bosporus means in Greek “ox ford” or “ox passage”; the name comes from a Greek myth about Io’s travels after Zeus turned her into an ox for her protection. The ancient Greeks referred to this strait as the Thracian Bosporus, as they called the Strait of Kerch the Cimmerian Bosporus. They also called a land area near these two straits by the same name: the Thracian Chersonesus, which is known today as Gallipoli, and the Cimmerian Chersonesus, known today as the Crimea. Due to the importance of the strait for the defense of Istanbul, the Ottoman sultans constructed a fortification on each side of it, Anadoluhisari (1393) and Rumelihisari (1451). Its strategic importance remains high: several international treaties have governed vessels using the waters. including the Montreux Convention Regarding the Regime of the Turkish Straits, signed in 1936. Some have argued that a massive flood occurring in the region around 5600 BC is the historic basis for the flood stories in the Epic of Gilgamesh and the Bible.

Under the Montreux Convention of 1936, commercial shipping has the right of free passage through the Straits in peacetime, although Turkey claims the right to impose regulations for safety and environmental purposes. In October 2002, Turkey placed new restrictions on oil tanker transit through the Bosporus that have slowed tanker transit, including a ban on nighttime transit for ships longer than 200 meters, effectively including all crude oil and large petroleum product tankers. Poor weather has caused transit delays as well; during the past winter, delays reportedly reached as much as 20 days for oil tankers waiting to transit the Straits, costing about $50,000 in demurrage charges per day.