ocean:Usually with the. The vast continuous body of salt water covering the greater part of the earth's surface and surrounding its land masses; the sea, esp. the open sea. (In early times, when only the one great mass of land, the Eastern hemisphere, with its islands, was known, the ocean was the ‘Great Outer Sea’ of boundless extent, everywhere surrounding the land, as opposed to the Mediterranean and other inland seas.)——Oxford English Dictionary

ocean:one of the very large areas of sea on the Earth’s surface.——Longman Dictionary of Contemporary English  @en