Oceans (from Okeanos in Greek, the ancient Greeks noticing the strong current that flowed off Gibraltar and assuming it was a great river) cover almost three quarters (71%) of the surface of the Earth, and nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep. The area of the oceans is 361 million sq km.

This global, interconnected body of salt water, called the World Ocean, is generally divided by the continents and archipelagos into the following bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean, and the Arctic Ocean.

Smaller regions of the oceans are called seas, gulfs, straits and other names.

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 where there are no continents. From this point of view, there are three "oceans" today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own "ocean", being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the strait off entirely, making the Mediterranean a fourth "ocean". (The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect 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.)