The Byzantines, one of the oldest settlers on the Bosphorus, called it Bosporos (Greek: Βόσπορος).
This word was derived from the combination of the names βοῦς (bous), meaning cow or ox, and πόρος (poros), meaning road and passage.
The name of Bosporos, which means ox or cow crossing, is based on the fact that Zeus, in Greek mythology, was in love with a girl named Io. According to the story, Io was the daughter of Inahos, the god of rivers. When Zeus, the king of the gods, saw this beautiful girl, he fell in love with him and secretly began to sleep with her husband Hera. One day, as she is about to catch Hera, she turns herself into a cloud and Io turns into a cow. Hera, who is not deceived, asks the cow as a gift from her husband. In order to keep him away from Zeus, he leaves him under the supervision of a monster named Argos Panoptis. However, Zeus sends Hermes to Argos. Hera then haunts him with a fly to disturb Io who has become a cow. When Io comes to the throat with the power to get rid of the fly, he leaves himself to the waters of the throat and swims through this obstacle.
She gives birth to a girl named Keroessa on the coast and when she grows up she marries Poseidon, the god of the sea, and gives birth to a boy named Byzas. This child founded the city of Byzantion where he was named after him. These mythological stories are important because they give names to both the city of Istanbul and the Bosphorus.
Another opinion on the origin of Bosporus, one of the names used in the ancient times of the Bosphorus, is that the word comes from Fosforos (Greek: Φωσφόρος - phosphorous, luminous).
The Bosphorus is still known in western languages by this name or its different forms. In the old Turkish sources, Halic-i bahr-i rûm (Strait of Marmara Sea), Halic-i bahr-i black (Black Sea Strait), Halic-i konstantiniyye (Constantine Strait), Merecü'l bahreyn / Mecma'ül bahreyn (where the two seas meet) and the Islamic Strait.