An ocean is a body of saline water that composes a large part of a planet’s hydrosphere. In the context of Earth, it refers to one or all of the major divisions of the planet’s World Ocean – they are, in descending order of area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. The word “sea” is often used interchangeably with “ocean”, but strictly speaking a sea is a body of saline water (possibly a division of the World Ocean) partly or fully enclosed by land.

Earth is the one of the only planets known to have an ocean (or any large amounts of open liquid water). Approximately 72% of the planet’s surface is covered by saline water that is customarily divided into several principal oceans and smaller seas, with the ocean covering approximately 71% of the Earth’s surface.