An ocean (from Greek , Okeanos (Oceanus)) is a major body of seawater, and a principal component of the hydrosphere. Approximately 72% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. More than half of this area is 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 30 to 38 ppt.

Overview

Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean."Distribution of land and water on the planet". ''UN Atlas of the Oceans This concept of a global ocean as 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. These divisions are (in descending order of size):

• Pacific Ocean
• Atlantic Ocean
• Indian Ocean
• Southern Ocean, sometimes subsumed as the southern portions of the Pacific, Atlantic, and Indian Oceans
• Arctic Ocean, sometimes considered a sea of the Atlantic

The Pacific and Atlantic may be further subdivided by the equator into northern and southern portions. Smaller regions of the oceans are called seas, gulfs, bays and other names. There are also some smaller bodies of saltwater that are on land and not interconnected with the World Ocean, such as the Aral Sea and the Great Salt Lake – though they may be referred to as 'seas', they are actually salt lakes.

As ocean covers most of Earth's surface 1 it has a significant effect on the biosphere. The evaporation of ocean water (as a component of the water cycle) is the source of most rainfall, and ocean temperatures determine climate and wind patterns. Life within the ocean had already evolved 3 billion years prior to the occurrence of animal and plant life on land. The ocean is home to many species of life throughout all aquatic layers. In addition, the amount of life and distance from the shore (abiotic component) affect the major distribution of marine biomes.

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 perspective, there are three oceans today: the World Ocean and the Caspian and Black Seas, the latter two having been 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.