noun
northern sea; northern ocean
General term for a sea or ocean in the north. Can refer to any northern sea, not a specific proper name.
その船は北海を航海している。
The ship is sailing the northern seas.
noun
North Sea
Specifically the North Sea between Great Britain and continental Europe.
The North Sea oil fields are important for Europe's energy supply.
noun, noun, used as a prefix
Abbreviation of 北海道. Used in compound words like 北海タイムス or as a prefix in names.
See also: 北海道
北海タイムスは北海道の新聞だ。
The Hokkai Times is a newspaper in Hokkaido.