An ocean is a body of saline water that composes a large part of a planet's hydrosphere. In the context of Earth, it also refers to major divisions of the planet's World Ocean, such as the Atlantic Ocean. The word "sea" is often used interchangeably with "ocean", but a sea is a body of saline water (possibly a division of the World Ocean) in a more inland location.

Earth's global ocean is the largest confirmed surface ocean on all observable planets. Approximately 71% of the planet's surface (3.6x10⁸ km²) is covered by saline water that is customarily divided into several principal oceans and smaller seas. Because it is the principal component of Earth's hydrosphere, the world ocean is integral to all known life, forms part of the carbon cycle, and influences climate and weather patterns. The total volume is approximately 1.3 billion cubic kilometres (310 million cu mi) with an average depth of 3,790 metres (12,430 ft). It is the habitat of 230,000 species known to science, however much of the ocean's depths remain unexplored and it is estimated that over two million marine species may exist. The origin of Earth's oceans is still unknown though they are believed to have first appeared in the Hadean period and may have been the point of origin for the emergence of life.

Extraterrestrial oceans may be composed of a wide range of elements and compounds. The only confirmed large stable bodies of extraterrestrial surface liquids are the lakes of Titan though there is evidence for the existence of oceans elsewhere in the Solar System. Early in their geologic histories, Mars and Venus are theorised 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, though the water on Mars is no longer oceanic, and a runaway greenhouse effect may have boiled away the global ocean of Venus. Compounds such as salts and ammonia mixed water are known to lower the freezing point potentially allowing water to exist in large quantities in extraterrestrial environments as brine or convecting ice. The Solar System's gas giant planets are also believed to possess liquid atmospheric layers of yet to be confirmed compositions. Unconfirmed oceans are speculated beneath the surface of many dwarf planets and natural satellites, notably the ocean of Europa is believed to have over twice the water volume of Earth. Oceans may also exist on exoplanets and exomoons, including surface oceans of liquid water within a circumstellar habitable zone. Ocean planets are a hypothetical type of planet with a surface completely covered with liquid.