The borders of the oceans are the limits of Earth's oceanic waters. The definition and number of oceans can vary depending on the adopted criteria. The principal divisions (in descending order of area) of the five oceans are the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays, straits, and other terms. Geologically, an ocean is an area of oceanic crust covered by water.

Overview Though generally described as several separate oceans, the world's oceanic waters constitute one global, interconnected body of salt water sometimes referred to as the World Ocean or Global Ocean." This concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.

The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. The principal divisions (in descending order of area) are the: Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays, straits, and other terms.

Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle. Continental crust is thicker but less dense. From this perspective, the Earth has three oceans: the World Ocean, the Caspian Sea, and the Black Sea. The latter two were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is at times a discrete ocean because tectonic plate movement has repeatedly broken its connection to the World Ocean through the Strait of Gibraltar. The Black Sea is connected to the Mediterranean through the Bosporus, but the Bosporus is a natural canal cut through continental rock some 7,000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.

Despite their names, some smaller landlocked "seas" are not connected with the World Ocean, such as the Caspian Sea (which is nevertheless, geologically, itself a full-fledged ocean—see above) and numerous salt lakes such as the Aral Sea.

A complete hierarchy showing which seas belong to which oceans, according to the International Hydrographic Organization (IHO), is available at the European Marine Gazetteer website.European Marine Gazetteer browser, MarBEF Data System – European Marine Gazetteer See also the list of seas article for the seas included in each ocean area. Also note there are many varying definitions of the world's seas and no single authority.

List of oceans The world ocean is divided into a number of principal oceanic areas that are delimited by the continents and various oceanographic features: these divisions are the Atlantic Ocean, Arctic Ocean (sometimes considered an estuary of the Atlantic), Indian Ocean, Pacific Ocean, and the Southern Ocean, defined by the International Hydrographic Organization (IHO) in 2000, the latter being a relatively new addition identified with a distinct ecosystem and a unique impact on global climate. In turn, oceanic waters are interspersed by many smaller seas, gulfs, and bays.

If viewed from the southern pole of Earth, the Atlantic, Indian, and Pacific Oceans can be seen as lobes extending northward from the Southern Ocean. Farther north, the Atlantic opens into the Arctic Ocean, which is connected to the Pacific by the Bering Strait, forming a continuous expanse of water.

The five oceans are: The Pacific Ocean, the largest of the oceans, also reaches northward from the Southern Ocean to the Arctic Ocean. It spans the gap between Australia and Asia, and the Americas. The Pacific Ocean meets the Atlantic Ocean south of South America at Cape Horn. The Atlantic Ocean, the second largest, extends from the Southern Ocean between the Americas, and Africa and Europe, to the Arctic Ocean. The Atlantic Ocean meets the Indian Ocean south of Africa at Cape Agulhas. The Indian Ocean, the third largest, extends northward from the Southern Ocean to India, the Arabian Peninsula, and Southeast Asia in Asia, and between Africa in the west and Australia in the east. The Indian Ocean joins the Pacific Ocean to the east, near Australia at South East Cape. The Arctic Ocean is the smallest of the five. It joins the Atlantic Ocean near Greenland and Iceland and joins the Pacific Ocean at the Bering Strait. It overlies the North Pole, touching North America in the Western Hemisphere and Scandinavia and Siberia in the Eastern Hemisphere. The Arctic Ocean is partially covered in sea ice, the extent of which varies according to the season. The Southern Ocean is a proposed ocean surrounding Antarctica, dominated by the Antarctic Circumpolar Current, generally the ocean south of 60 degrees south latitude. The Southern Ocean is partially covered in sea ice, the extent of which varies according to the season. The Southern Ocean is the second smallest of the five named oceans.

Arctic Ocean

The Arctic Ocean covers much of the Arctic and washes upon Northern America and Eurasia. It is sometimes considered a sea or estuary of the Atlantic Ocean.The Ocean, The MarineBio Conservation SocietyArctic Ocean, Encyclopædia Britannica

The International Hydrographic Organization (IHO) defines the limits of the Arctic Ocean (excluding the seas it contains) as follows:Between Greenland and West Spitsbergen – The Northern limit of Greenland Sea.Defined by IHO as "A line joining the Northernmost point of Spitsbergen [Svalbard] to the Northernmost point of Greenland."

Between West Spitsbergen and North East Land – the parallel of lat. 80° N.

From Cape Leigh Smith to Cape Kohlsaat – the Northern limit of Barents Sea.Defined by IHO as "Cape Leigh Smith across the Islands Bolshoy Ostrov (Great Island) [Storøya], Gilles [Kvitøya] and Victoria; Cape Mary Harmsworth (Southwestern extremity of Alexandra Land) along the northern coasts of Franz-Josef Land as far as Cape Kohlsaat (81°14′N, 65°10′E)."

From Cape Kohlsaat to Cape Molotov – the Northern limit of Kara Sea.Defined by IHO as "Cape Kohlsaat to Cape Molotov (81°16′N, 93°43′E) (Northern extremity of Severnaya Zemlya on Komsomolets Island)."

From Cape Molotov to the Northern extremity of Kotelni Island – the Northern limit of Laptev Sea.Defined by IHO as "A line joining Cape Molotov to the Northern extremity of Kotelni Island (76°10′N, 138°50′E)."

From the Northern extremity of Kotelni Island to the Northern point of Wrangel Island – the Northern limit of East Siberian Sea.Defined by IHO as "A line from the Northernmost point of Wrangel Island (179°30'W) to the Northern sides of the De Long Islands (including Henrietta and Jeannette Islands) and Bennett Island, thence to the Northern extremity of Kotelni Island."

From the Northern point of Wrangel Island to Point Barrow – the Northern limit of Chuckchi Sea.Defined by IHO as "A line from Point Barrow, Alaska (71°20′N, 156°20′W) to the Northernmost point of Wrangel Island (179°30'W)."

From Point Barrow to Cape Land's End on Prince Patrick Island – the Northern limit of Beaufort Sea,Defined by IHO as "A line from Point Barrow, Alaska, to Lands End, Prince Patrick Island (76°16′N, 124°08′W)." through the Northwest coast of Prince Patrick Island to Cape Leopold M'Clintock, thence to Cape Murray (Brook Island [sic]) and along the Northwest coast to the extreme Northerly point; to Cape Mackay (Borden Island); through the Northwesterly coast of Borden Island to Cape Malloch, to Cape Isachsen (Ellef Ringnes Island); to the Northwest point of Meighen Island to Cape Stallworthy (Axel Heiberg Island) to Cape Colgate the extreme West point of Ellesmere Island; through the North shore of Ellesmere Island to Cape Columbia thence a line to Cape Morris Jesup (Greenland).Note that these definitions exclude any marginal waterbodies that are separately defined by the IHO (such as the Kara Sea and East Siberian Sea), though these are usually considered to be part of the Arctic Ocean.

The CIA defines the limits of the Arctic Ocean differently, as depicted in the map comparing its definition to the IHO's definition.

Notes

Atlantic Ocean

The Atlantic Ocean separates the Americas from Europe and Africa. It may be further subdivided by the Equator into northern and southern portions.

North Atlantic Ocean The 3rd edition, currently in force, of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas defines the limits of the North Atlantic Ocean (excluding the seas it contains) as follows:On the West. The Eastern limits of the Caribbean Sea,Defined by IHO as "From Point San Diego (Puerto Rico) Northward along the meridian thereof (65°39'W) to the 100 fathom line, thence Eastward and Southward, in such a manner that all islands, shoals and narrow waters of the Lesser Antilles are included in the Caribbean Sea as far as Galera Point (Northeast extremity of the island of Trinidad). From Galera Point through Trinidad to Galeota Point (Southeast extreme) and thence to Baja Point () in Venezuela". the Southeastern limits of the Gulf of MexicoDefined by IHO as "A line joining Cape Catoche Light () with the Light on Cape San Antonio in Cuba, through this island to the meridian of 83°W and to the Northward along this meridian to the latitude of the South point of the Dry Tortugas (24°35'N), along this parallel Eastward to Rebecca Shoal (82°35'W) thence through the shoals and Florida Keys to the mainland at eastern end of Florida Bay, all the narrow waters between the Dry Tortugas and the mainland being considered to be within the Gulf". from the North coast of Cuba to Key West, the Southwestern limit of the Bay of FundyDefined by IHO as "A line running northwesterly from Cape St. Mary (44°05'N) Nova Scotia, through Machias Seal Island (67°06'W) and on to Little River Head (44°39'N) in the State of Maine". and the Southeastern and Northeastern limits of the Gulf of St. Lawrence.Defined by IHO as "A line from Cape Canso () to Red Point () in Cape Breton Island, through this Island to Cape Breton and on to Pointe Blanche () in the Island of St. Pierre, and thence to the Southwest point of Morgan Island ()" and "A line running from Cape Bauld (North point of Kirpon Island, ) to the East extreme of Belle Isle and on to the Northeast Ledge (). Thence a line joining this ledge with the East extreme of Cape St. Charles (52°13'N) in Labrador".

On the North. The Southern limit of Davis StraitDefined by IHO as "The parallel of 60° North between Greenland and Labrador". from the coast of Labrador to Greenland and the Southwestern limit of the Greenland SeaDefined by IHO as "A line joining Straumness (NW extreme of Iceland) to Cape Nansen () in Greenland". and Norwegian SeaDefined by IHO as "From a point on the West coast of Norway in Latitude 61°00' North along this parallel to Longitude 0°53' West thence a line to the NE extreme of Fuglö () and on to the East extreme of Gerpir () in Iceland". from Greenland to the Shetland Islands.