Ocean, the continuous body of saltwater that covers about 70 per cent of the earth's surface. The term “ocean” also refers to any of the four major subdivisions of this body. These are, in order of size, the Pacific, Atlantic, Indian, and Arctic oceans. The Pacific Ocean contains about half of the total ocean area and is larger in area than all the world's land combined. The oceans account for 97 per cent of the earth's water area.

The Equator divides the Atlantic and Pacific oceans into North and South Atlantic and North and South Pacific. Occasionally, but incorrectly, the southernmost part of the Atlantic, Pacific, and Indian oceans is called the Antarctic Ocean.

“Sea” is a term commonly used to designate (1) the ocean, (2) a subdivision of the ocean, or (3) a salt lake lacking an outlet to the ocean (for example, the Dead Sea and the Caspian Sea). In this article the word “sea” means the same as “ocean” unless otherwise specified.

Under international law a nation owns its territorial (coastal) waters, which extend 12 nautical miles beyond its coast. (One nautical mile is equal to 1.151 statute [land] miles or 1.852 kilometers.) All ships may move freely outside that area. Each nation also has exclusive rights to all marine life in waters extending 200 nautical miles beyond its shores. The use and exploitation of the ocean is governed by the Law of the Sea Treaty (1982).

Oceanography (or oceanology) is the science covering all aspects of ocean study and exploration. It draws on the sciences of botany, zoology, meteorology, physics, chemistry, geology, fluid mechanics, and applied mathematics. Marine biology, biological oceanography, is the branch of biological science concerned with plant and animal life in the sea. Many of the problems associated with the ocean are so complex that they must be studied by a team of specialists in various sciences.