The World Ocean, world ocean, or global ocean, is the interconnected system of the Earth's oceanic (or marine) waters, and comprises the bulk of the hydrosphere, covering almost 71% of the Earth's surface, with a total volume of 1.332 billion cubic kilometers.

The unity and continuity of the World Ocean, with relatively free interchange among its parts, is of fundamental importance to oceanography. It is divided into a number of principal oceanic areas that are delimited by the continents and various oceanographic features: these divisions are the Atlantic Ocean, Arctic Ocean (rarely considered a sea of the Atlantic), Indian Ocean, Pacific Ocean, and Southern Ocean (typically reckoned instead as just the southern portions of the Atlantic, Indian, and Pacific Oceans). In turn, oceanic waters are interspersed by many smaller seas, gulfs, and bays.

A global ocean has existed in one form or another on Earth for eons, and the notion dates back to classical antiquity (in the form of Oceanus). The contemporary concept of the World Ocean was coined by the Russian oceanographer Yuly Shokalsky in the early 20th century to describe what is basically a solitary, continuous ocean that covers and encircles most of the Earth.^{[citation needed]}