The North Sea is a marginal sea of the Atlantic Ocean located between Great Britain, Scandinavia, Germany, the Netherlands, and Belgium. An epeiric (or "shelf") sea on the European continental shelf, it connects to the ocean through the English Channel in the south and the Norwegian Sea in the north. It is more than 970 kilometres (600 mi) long and 580 kilometres (360 mi) wide, with an area of around 750,000 square kilometres (290,000 sq mi).

The North Sea has long been the site of important European shipping lanes as well as a major fishery. The sea is a popular destination for recreation and tourism in bordering countries and more recently has developed into a rich source of energy resources including fossil fuels, wind, and early efforts in wave power.

Historically, the North Sea has featured prominently in geopolitical and military affairs, particularly in Northern Europe but also globally through the power northern European actors projected worldwide during much of the Middle Ages and modern era. The North Sea was the centre of the Vikings' rise and subsequently, the Hanseatic League, the Netherlands, and the British each sought to dominate the North Sea and through it to control access to the markets and resources of the world. As Germany's only outlet to the ocean, the North Sea continued to be strategically important through both World Wars.

North is a noun, adjective, or adverb indicating direction or geography. It is one of the most important directions.

North is one of the four cardinal directions or compass points. It is the opposite of south and is perpendicular to east and west.

By convention, the top side of a map is north.

To go north using a compass for navigation, set a bearing or azimuth of 0° or 360°.

North is specifically the direction that, in Western culture, is treated as the fundamental direction:

The word north is related to the Old High German nord, both descending from the Proto-Indo-European unit ner-, meaning "down" (or "under"). (Presumably a natural primitive description of its concept is "to the left of the rising sun".)

Latin borealis is from Greek boreas "north wind, north", in mythology (according to Ovid) personified as the son of the river-god Strymon, and father of Calais and Zetes; septentrionalis is from septentriones, "the seven plow oxen", a name of Ursa Maior. Greek arktikos "northern" is named for the same constellation (cf. Arctic).

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean. It is also used sometimes to describe a large saline lake that lacks a natural outlet, such as the Caspian Sea.

Arctic (belonging to the Arctic Ocean) and Antarctic (Southern Ocean) seas, as well as some other seas freeze in winter. This occurs below the freezing point of pure water, at about -1.8 °C (28.8 °F). Frozen salt water becomes sea ice.

Humans navigated seas from antiquity. Ancient Egyptians and Phoenicians navigated the Mediterranean Sea and the Red Sea. Hannu was the first see explorer of whom there is any information. He sailed along the Red Sea and further to the Arabian Peninsula and the African Coast around 2750 BC. In the 1st millennium BC, Phoenicians and Greeks established colonies all over the Mediterranean, including its outlets like the Black Sea. The seas along the eastern and the southern Asian coast were used by Arabs and Chinese for navigation, and the North Sea and the Baltic Sea were known to Europeans in Roman times. Other seas were not used for navigation in the antiquity and were actually discovered.