An ocean (from Ancient Greek Ὠκεανὸς (Okeanos); the World Ocean of classical antiquity) 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 only planet known to have an ocean (or any large amounts of open liquid water). Approximately 72% of the planet's surface (~3.6x108 km2) 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. In terms of the hydrosphere of the Earth, the ocean contains 97% of the Earth's water. Oceanographers have stated that out of 97%, only 5% of the ocean as a whole on Earth has been explored. 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 3,682 metres (12,080 ft). It is the habitat of 230,000 known species, although much of the ocean's depths remain unexplored and it is estimated that over two million marine species exist. The origin of Earth's oceans is still unknown, but oceans are believed to have formed in the Hadean period and may have been the impetus for the emergence of life.
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106,400,000 square kilometres (41,100,000 sq mi), it covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. The first part of its name refers to Atlas of Greek mythology, making the Atlantic the "Sea of Atlas".
The oldest known mention of "Atlantic" is in The Histories of Herodotus around 450 BC (Hdt. 1.202.4): Atlantis thalassa (Greek: Ἀτλαντὶς θάλασσα; English: Sea of Atlas). The term Ethiopic Ocean, derived from Ethiopia, was applied to the southern Atlantic as late as the mid-19th century. Before Europeans discovered other oceans, the term "ocean" itself was synonymous with the waters beyond the Strait of Gibraltar that we now know as the Atlantic. The early Greeks believed this ocean to be a gigantic river encircling the world.
The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Eurasia and Africa to the east, and the Americas to the west. As one component of the interconnected global ocean, it is connected in the north to the Arctic Ocean (which is sometimes considered a sea of the Atlantic), to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south. (Other definitions describe the Atlantic as extending southward to Antarctica.) The equator subdivides it into the North Atlantic Ocean and South Atlantic Ocean.