An ocean (from Okeanos (Oceanus) in Greek) is a principal component of the hydrosphere: a major body of saline water that, in totality, covers about 71% of the Earth's surface (or an area of some 361 million square kilometers). Nearly half of the world's marine (oceanic) waters are over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 31 to 38 ppt.

Though somewhat arbitrarily divided into several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean. The major oceanic divisions are defined in part by the continents, various archipelagos, and a number of other criteria; these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean (which is sometimes reckoned instead as just the southern portions of the Pacific, Atlantic, and Indian Oceans), and the Arctic Ocean (which is sometimes considered a sea of the Atlantic). Smaller regions of the oceans are called seas, gulfs, bays and other names.

There are also some smaller bodies of salt water that are inland and not interconnected with the World Ocean: e.g., the Caspian Sea, Aral Sea, the Great Salt Lake. These are not considered to be oceans or parts of oceans, though some are called 'seas'.

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, the Caspian and the Black Seas, the latter two of which were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly a discrete 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. 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. Status OK.