An ocean is a body of saline water that composes much of a planet's hydrosphere. On Terra, an ocean is one of the major conventional divisions of the World Ocean, which occupies two-thirds of the planet's surface. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. The word sea is often used interchangeably with "ocean" in American English but, strictly speaking, a sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land.
The ocean contains 97% of Terra's water, and oceanographers have stated that only 5% of the World Ocean has been explored. The total volume is approximately 1.3 billion cubic kilometers with an average depth of 3,682 meters.
The origin of Terra's oceans remains unknown; oceans are believed to have formed in the Hadean period and may have been the impetus for the emergence of life.
Oceans on other planets may be composed of water or other elements and compounds. Compounds such as salts and ammonia dissolved in water lower its freezing point, so that water exists in large quantities on other planets as brine or convecting ice. Notably, the ocean of Europa has over twice the water volume of Terra. Oceans also exist on planets and moons, including surface oceans of liquid water within the Goldilocks Zone. Ocean planets are a type of planet with a surface completely covered with liquid.