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What is the difference between an ocean and a sea?

Although the terms "ocean" and "sea" are often used interchangeably, a sea is generally considered to be smaller than an ocean. One definition of "ocean" is: a great body of interconnecting salt water that covers 71 percent of the Earth's surface. There are four major oceans—Arctic, Atlantic, Indian, and Pacific. However, some sources do not include the Arctic Ocean, calling it a marginal sea. The term "sea" is often assigned to saltwater areas on the margins of oceans, such as the Mediterranean Sea situated beside the Atlantic Ocean.

Sources: Mayhew, Susan, and Anne Penny. The Concise Oxford Dictionary of Geography, p. 164; The Guinness Book of Answers, 8th ed., pp. 62-65; The Universal Almanac 1992, p. 296.

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