On Earth, an ocean is one or all of the major divisions of the planet's World Ocean--which are, in descending order of area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.
On Earth, an ocean is one of the major conventional divisions of the World Ocean, which occupies two-thirds of planet's surface. These are, in descending order by area, the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans.