The word "ocean" is often used interchangeably with "sea" in American English.
Strictly speaking, a sea is a body of saline water (generally a division of the world ocean) partly or fully enclosed by land, As the world ocean is the principal component of Earth's hydrosphere, it is integral to life, forms part of the carbon cycle, and influences climate and weather patterns.
The World Ocean is the habitat of 230,000 known species, but because much of it is unexplored, the number of species that exist in the ocean is much larger, possibly over two million.
The origin of Earth's oceans is unknown; oceans are thought to have formed in the Hadean eon and may have been the impetus for the emergence of life.
Extraterrestrial oceans may be composed of water or other elements and compounds.
The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan, although there is evidence for the existence of oceans elsewhere in the Solar System.
Early in their geologic histories, Mars and Venus are theorized to have had large water oceans.
The Mars ocean hypothesis suggests that nearly a third of the surface of Mars was once covered by water, and a runaway greenhouse effect may have boiled away the global ocean of Venus.
Compounds such as salts and ammonia dissolved in water lower its freezing point so that water might exist in large quantities in extraterrestrial environments as brine or convecting ice.
Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites; notably, the ocean of Europa is estimated to have over twice the water volume of Earth.
The Solar System's giant planets are also thought to have liquid atmospheric layers of yet to be confirmed compositions.
Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone.