The phrase is used in reference to sailors and pirates in the arts and popular culture and can be associated with the Mediterranean Sea, and/or the Arabian Seven Seas east of Africa and India (as told with Sinbad's seven journeys, and Captain Kidd), or is sometimes applied to the Caribbean Sea and seas around the Americas (with pirates such as Blackbeard).
The Seven Seas was part of the vernacular of several nations long before some of the oceans named were known to the inhabitants of Europe and Asia, and is referred to in the literature of the ancient Hindus, Chinese, Persians, Romans and other nations.
The Arabs and their near neighbours considered the Seven Seas (Arabic: البحار السبعة) to be the seas that they encountered in their voyages to The East. They were trading routes in ancient times and since the time of Muhammad, they are the places where Islam spread and is widely practised.
In the 9th century AD, author Ya'qubi wrote:
Whoever wants to go to China must cross seven seas, each one with its own color and wind and fish and breeze, completely unlike the sea that lies beside it. The first of them is the Sea of Fars, which men sail setting out from Siraf. It ends at Ra’s al-Jumha; it is a strait where pearls are fished. The second sea begins at Ra’s al-Jumha and is called Larwi. It is a big sea, and in it is the Island of Waqwaq and others that belong to the Zanj. These islands have kings. One can only sail this sea by the stars. It contains huge fish, and in it are many wonders and things that pass description. The third sea is called Harkand, and in it lies the Island of Sarandib, in which are precious stones and rubies. Here are islands with kings, but there is one king over them. In the islands of this sea grow bamboo and rattan. The fourth sea is called Kalah and is shallow and filled with huge serpents. Sometimes they ride the wind and smash ships. Here are islands where the camphor tree grows. The fifth sea is called Salahit and is very large and filled with wonders. The sixth sea is called Kardanj; it is very rainy. The seventh sea is called the sea of Sanji, also known as Kanjli. It is the sea of China; one is driven by the south wind until one reaches a freshwater bay, along which are fortified places and cities, until one reaches Khanfu.
This passage demonstrates the Seven Seas as referenced in Medieval Arabian literature:
In British Colonial times the Clipper Ship Tea Route from China to England was the longest trade route in the world. It took sailors through seven seas near the Dutch East Indies: the Banda Sea, the Celebes Sea, the Flores Sea, the Java Sea, the South China Sea, the Sulu Sea, and the Timor Sea. The Seven Seas referred to those seas, and if someone had sailed the Seven Seas it meant he had sailed to, and returned from, the other side of the world.
Ancient texts and legends speak of Seven Seas or Sapta Sindhu (Sopto Sindhu). Sindhu and Sagar/Sagor mean sea in different languages of the subcontinent. According to the Vishnu Purana, the seas were Lavana/Lobon (salt), Iksu/Ikkhu (sugar-cane), Sura/Sura (wine), Sarpi (clarified butter or Ghee), Dadhi/Dodhi (yoghurt or curd), Dugdha/Dugdho (milk) and Jala/Jol (water). There may be variant list of names. For example, there is also mention of a Peeta/Pit Sagara/Sagor ie Yellow Sea. The word Pita means yellow. The Kshira Sagar or Kshir Sagar or Khir Sagor is also the white Sea. The word Kshira/Khir literally means condensed milk.. However, it also denotes white like milk.
The meaning of septem maria (Latin) in Ancient Rome is different than the phrase "seven seas" in the modern era. The navigable network in the mouths of the Po river discharges into saltmarshes on the Adriatic shore and was colloquially called the "Seven Seas" in ancient Roman times. Pliny the Elder, a Roman author and fleet commander, wrote about these lagoons, separated from the open sea by sandbanks:
All those rivers and trenches were first made by the Etruscans, thus discharging the flow of the river across the marshes of the Atriani called the Seven Seas, with the famous harbor of the Etruscan town of Atria which formerly gave the name of Atriatic to the sea now called the Adriatic.
A history of Venice states:
The expression "to sail the seven seas" was a classical flourish signifying nautical skill. It was applied to the Venetians long before they sailed the oceans.
When R. Dimi came he said R. Yohanan said: "What is the meaning of the verse, 'For he hath founded it upon the seas and established it upon the floods.' (Ps. 24:2)? This refers to the seven seas and four rivers that surround the land of Israel. And what are the seven seas? The sea of Tiberias, the Sea of Sodom, the Sea of Helath, the Sea of Hiltha, the Sea of Sibkay, the Sea of Aspamia and the Great Sea. And what are the four rivers? The Jordan, the Yarmuk, the Keramyhon and Pigah."
According to this and other passages, the Talmudic Seven Seas include:
Various transliterations for the sea names from Hebrew exist. For Helath: Chelath and Shelyith. For Hiltha: Chiltha and Chultha. For Sibkay: Sibchi and Somcho. And for Aspamia: Apamia.
The 17th century churchman and scholar John Lightfoot mentions this set of seas in his Commentary on the New Testament. A chapter titled The Seven Seas according to the Talmudists, and the four Rivers compassing the Land includes the "Great Sea" (now called the Mediterranean Sea), the "Sea of Tiberias" (Sea of Galilee), the "Sea of Sodom" (Dead Sea), the "Lake of Samocho" (probably the (mostly) dried-up Hula Lake, called Semechonitis by Josephus and lake Sumchi in the Talmud), also called the "Sibbichaean". Lightfoot does not comment on the remaining three seas.
After the discovery of the Americas during the Age of Discovery, the "seven seas" were reckoned by some as:
The last two of these are now reckoned to be part of the Atlantic Ocean, and the Mediterranean either part of the Atlantic or omitted. Splitting the Atlantic and Pacific into north and south and adding the Southern Ocean returns the list to seven.
Detailed reckonings of the divisions of the world ocean into oceans and seas is not limited to lists of seven. For example, the International Hydrographic Organization recognizes many marginal seas; some saltwater lakes and the freshwater Sea of Galilee also have "sea" in their names.
Popular expression for all of the world’s oceans.
the seven seas: 2. All the oceans of the world (conventionally listed as the Arctic, Antarctic, North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Oceans).