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Seas and Oceans –what’s the  difference

Map showing the location of the Sargasso Sea. Image: MediaWiki Commons
Salt Water or Fresh Water

In general, as explains, the words ocean and sea are often used interchangeably to refer to the enormous body of salt water that covers most of Earth. The Earth is primarily an ocean planet, 71 per cent of which is covered in water. Technically speaking, an ocean is one of the big five (or seven) divisions of this expanse (like the Atlantic and the Pacific), while a sea is a smaller portion of this (such as the Mediterranean, the Caribbean, the Arabian Sea, the South China Sea, and the Red Sea, and many others) – typically one that is bounded in some way by smaller landmasses.


  • Some bodies of water are surrounded by land, but they are big enough to be considered seas, such as the Black Sea.
  • The relatively calm portion of the Atlantic Ocean known as the Sargasso Sea (see below) is not bounded by any land but is instead defined by its location between ocean currents.
  • Not every body of water considered to be a sea has the word sea in its name. The Gulf of Mexico and the Bay of Bengal fit the criteria to be regarded as seas. Hudson Bay is considered an inland sea.
  • Not all water expanses with sea in its name is a sea at all – the Caspian Sea and the Dead Sea are, in fact, saltwater lakes, as is the Sea of Galilee.

Sargasso Sea
The exception to the definition of a sea is the Sargasso Sea. The Sargasso Sea is a sea within the open ocean. It is the only sea in the world without a land boundary.

Map showing the location of the Sargasso Sea. Image: MediaWiki Commons
Attribution: Map showing the location of the Sargasso Sea with arrows showing the direction of the currents. Image: MediaWiki Commons, public domain

What is an ocean?
According to, when people say the ocean, they usually mean “the vast body of salt water that covers almost three fourths of Earth’s surface.” You can think of this as one big, unbounded body of water in which the continents are islands. This vast expanse—the world ocean, as it is sometimes called—has been divided into sections, roughly based on the position of each section between continents.

Each of these sections is called an ocean, and each has a specific name: the Pacific Ocean (from the east coasts of Asia and Australia to the west coasts of the Americas), the Atlantic Ocean (from the east coasts of the Americas to the west coasts of Europe and Africa), the Indian Ocean (between the east coast of Africa, the Indian subcontinent, and the west coast of Australia), the Arctic Ocean (in the extreme global north), and the Antarctic Ocean (in the extreme global south).

The Pacific and the Atlantic are the biggest oceans and are so big that they are further divided into the North Atlantic and South Atlantic; and the North Pacific and South Pacific. If you count these divisions, you will end up with a list of seven oceans (instead of five).

The world’s oceans supply at least half of the world’s oxygen and store about 50 times more carbon than the atmosphere. Oceans also influence the Earth’s climate through a constant transfer of heat from the Equator towards the poles. Evaporation from the ocean’s surface brings rain to much of the Earth’s land surfaces.

What is a sea?
Again explains what people mean when they say the sea. Often, it can mean the same thing as the ocean —the enormous, connected body of salt water that covers most of the planet. More specifically, though, a sea is “a division of these waters, of considerable extent, more or less definitely marked off by land boundaries.”

What are the seven seas?
In fact, there are more than fifty seas in the world. So why are we so used to hearing that there are seven (see above)?

Many geographers and historians believe that, in the ancient world, it most commonly referred to the Indian Ocean, the Black Sea, the Caspian Sea, the Adriatic Sea, the Persian Gulf, the Mediterranean Sea, and the Red Sea. But this likely varied in different parts of the world where other bodies of water were known.

How to use ocean vs sea
In the most general sense, sea and ocean are often used interchangeably to refer to the massive body of salt water covering most of the planet.

The technical distinction used for the purposes of geography is that an ocean is one of the five (or seven) divisions of these waters, while a sea is a smaller portion of the ocean, most often one bounded by land in some way.

Both sea and ocean are commonly used in phrases and compound words like seaside, oceanside, seawater, ocean liner, seascape, and many more. These terms are typically used in the general sense of the words or refer specifically to whatever body of water is nearby.

Both ocean and sea can also be used in similar figurative ways to refer to a large expanse, as in a sea of people, or a great amount, as in an ocean of possibilities. Sea is perhaps more commonly used in poetic ways.

In the example below, the Indian Ocean is shown as an open body of water.  The two areas of water that are partially enclosed by land are named the Red Sea and the Oman Sea.

Map showing the Indian Ocean, Oman Sea, and the Red Sea. Map: Epmistes, MediaWiki Commons. Attribution: Map showing the Indian Ocean, the Oman Sea, and the Red Sea. Map: Epmistes, MediaWiki Commons, public domain.

Other differences
The depth of an ocean is considerably greater than the sea. The deepest ocean in the world is the Pacific Ocean, with a depth of around 10,924 metres. By comparison, the deepest sea is the Caribbean Sea, with a depth of 6,946 metres. The area of an ocean is greater than that of a sea: for example, the Pacific Ocean covers an area of 60 million square miles – whereas the largest sea, the Mediterranean, covers an area of about 1.14 million square miles.

One more thing: the movement of currents in oceans affects the weather worldwide.

Earth’s smallest ocean is the Arctic Ocean, and the deepest is the Pacific Ocean. Various water movements distinguish the Antarctic (or Southern) Ocean from the Atlantic, Pacific, and Indian Oceans.

The oceans’ temperatures are different in the various parts of the world. For example, icebergs are formed by very cold waters near the poles, while waters around the Equator are relatively warm.

Seas are smaller than oceans and are usually located where the land and ocean meet. Typically, seas are partially enclosed by land. Then, just to confuse everything, there rivers: a large, flowing water body that merges into a sea or an ocean. The tributaries of a river are known as streams, creeks, and brooks.

Sources and Further Reading

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