1. A large expanse of salt water: The earth's five main oceans are the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans.

The oceans occupy huge regions of the earth's surface, and their boundaries are usually established by continental land masses and ridges in the floor or bottom of the oceans.

2. The entire body of salt water that covers more than 70 percent of the earth's surface: In science class, Greg learned about different oceans and that they covered a lot of the earth's outermost layer.
3. A vast amount or immense expanse of something, or an apparently unlimited space or quantity: As they drove back home, Jack and Jill saw an ocean of corn fields.

The Arctic Ocean is the smallest of the world's five oceans (after the Pacific Ocean, Atlantic Ocean, Indian Ocean, and the recently delimited [established boundaries] Southern Ocean): The Northwest Passage (U.S. and Canada) and Northern Sea Route (Norway and Russia) are two important seasonal waterways. A sparse network of air, ocean, river, and land routes circumscribes the Arctic Ocean.

The polar climate is characterized by persistent cold and relatively narrow annual temperature ranges; winters characterized by continuous darkness, cold and stable weather conditions, and clear skies; summers characterized by continuous daylight, damp and foggy weather, and weak cyclones with rain or snow.

The world's second-largest of the world's five oceans (after the Pacific Ocean, but larger than the Indian Ocean, Southern Ocean, and Arctic Ocean).

The Kiel Canal (Germany), Oresund (Denmark-Sweden), Bosporus (Turkey), Strait of Gibraltar (Morocco-Spain), and the Saint Lawrence Seaway (Canada-US) are important strategic access waterways.

The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth world ocean, the Southern Ocean, removed the portion of the Atlantic Ocean south of 60 degrees south latitude.

The Atlantic Ocean provides some of the world's most heavily trafficked sea routes between and within the Eastern and Western Hemispheres.

Other economic activities include the exploitation of natural resources, such as fishing, dredging of aragonite sands (The Bahamas), and production of crude oil and natural gas (Caribbean Sea, Gulf of Mexico, and North Sea).

The third largest of the world's five oceans (after the Pacific Ocean and Atlantic Ocean, but larger than the Southern Ocean and Arctic Ocean).

Four critically important access waterways are the Suez Canal (Egypt), Bab el Mandeb (Djibouti-Yemen), Strait of Hormuz (Iran-Oman), and Strait of Malacca (Indonesia-Malaysia).

The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth ocean, the Southern Ocean, removed the portion of the Indian Ocean south of 60 degrees south latitude.

The Indian Ocean floor is dominated by the Mid-Indian Ocean Ridge and subdivided by the Southeast Indian Ocean Ridge, Southwest Indian Ocean Ridge, and Ninetyeast Ridge.

A collective term for any form of energy that is extracted from the ocean; marine energy; ocean power; hydrokinetic energy: Ocean energy includes thermal energy from the difference between warmer surface waters and cooler deep waters, or mechanical energy from tides, waves, and currents.

Specialized engineering which deals with the applications of design, construction, and maintenance principles and techniques to an ocean environment: Ocean engineering helps people to operate with certainty beneath the surface of the sea in order to set up and to make use of marine resources.

The process of putting nutrients into the open ocean: Ocean fertilization has the purpose of increasing marine food in the water, for example with iron or urea, in order to boost phytoplankton growth and to remove carbon dioxide from the atmosphere.

A place where ecosystems are found at all levels, from the sunlit surface areas down to the deepest existence of life under water: "The ocean habitats cover about two-thirds of the Earth and support many kinds of life forms from the microscopic planktons to the blue whale, which is the biggest living mammal."

Various processes that operate in the ocean to equalize the distribution of heat, salt, various chemicals, and pollutants: Winds, currents and surface waves are all involved in ocean mixing to balance the circulation of different elements that enter the ocean waters at various rates and in different locations.

The rate of ocean mixing affects the extent to which carbon dioxide is exchanged between the atmosphere and the oceans.

One of the various techniques for extracting energy from the vertical temperature difference in the oceans: In principle, ocean thermal energy conversion can be used to generate electricity, desalinate water, support deep-water mariculture, and provide refrigeration and air-conditioning.

The temperature difference between warmer surface waters of the ocean and colder deep waters, whereas the deep waters are about 20-50 degrees Fahrenheit colder that the surface waters: In principle, the ocean thermal gradient can be utilized with various types of ocean thermal energy conversion systems.

A weather characteristic of continental margins and islands; marine climate: Maritime or oceanic climate is the Köppen categorization of weather conditions of those west coasts that show the annual temperature range of less than the average for that latitude because of the proximity of an ocean or sea.

The largest of the world's five oceans (followed by the Atlantic Ocean, Indian Ocean, Southern Ocean, and Arctic Ocean).

Strategically important access waterways include the La Perouse, Tsugaru, Tsushima, Taiwan, Singapore, and Torres Straits. The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth ocean, the Southern Ocean, removed the portion of the Pacific Ocean south of 60 degrees south.

Surface currents in the northern Pacific are dominated by a clockwise, warm-water gyre (broad circular system of currents) and in the southern Pacific by a counterclockwise, cool-water gyre.

In the northern Pacific, sea ice forms in the Bering Sea and the Sea of Okhotsk in winter; in the southern Pacific, sea ice from Antarctica reaches its northernmost extent in October. The ocean floor in the eastern Pacific is dominated by the East Pacific Rise, while the western Pacific is dissected by deep trenches, including the Mariana Trench, which is the world's deepest.

Endangered marine species include the dugong, sea lion, sea otter, seals, turtles, and whales.

A large body of recent oceanographic research has shown that the Antarctic Circumpolar Current (ACC), an ocean current that flows from west to east around Antarctica, plays a crucial role in global ocean circulation.

The region where the cold waters of the ACC meet and mingle with the warmer waters of the north defines a distinct border (the Antarctic Convergence) which fluctuates with the seasons, but which encompasses a discrete body of water and a unique ecologic region.

The Convergence concentrates nutrients, which promotes marine plant life, and which in turn allows for a greater abundance of animal life.

In the spring of 2000, the International Hydrographic Organization decided to delimit the waters within the Convergence as a fifth world ocean, or the Southern Ocean, by combining the southern portions of the Atlantic Ocean, Indian Ocean, and Pacific Ocean.

The Southern Ocean extends from the coast of Antarctica north to 60 degrees south latitude, which coincides with the Antarctic Treaty Limit and which approximates the extent of the Antarctic Convergence.

As such, the Southern Ocean is now the fourth largest of the world's five oceans (after the Pacific Ocean, Atlantic Ocean, and Indian Ocean, but larger than the Arctic Ocean). It should be noted that the inclusion of the Southern Ocean does not imply recognition of this feature as one of the world's primary oceans by the U.S. Government.