The words ocean and sea are often used interchangeably, but from a geographical view there is a difference. The main distinction between an ocean and a sea relates to size of the body of water and degree of enclosure. Oceans are vast, continuous bodies of saltwater, while seas are usually smaller and partially enclosed by land.

Oceans form the largest expanses of saltwater on Earth, spanning millions of square miles. They lie in the planet’s oceanic basins, which were formed due to tectonic plate movements. These basins filled with saltwater and separated the continents.

Seas often lie where the ocean meets land or exist within large gulfs and bays. They are partially enclosed by landmasses, like the Mediterranean Sea or Caribbean Sea. Some seas stand as distinct sub-regions of oceans. The Red Sea, for example, sits between Africa and the Arabian Peninsula.

Traditions have shaped how bodies of water are labeled. There is no standard that mandates which bodies of water must be called oceans versus seas.

Five Major Oceans

Oceans cover over 70% of the Earth’s surface. There are five major oceans:

• Pacific Ocean – covers an area of about 63 million sq mi (163 million sq km) and has an average depth of about 14,040 ft (4,280 m).
• Atlantic Ocean – covers an area of about 41 million sq mi (106 million sq km) and has an average depth of about 12,880 ft (3,930 m).
• Indian Ocean – covers an area of about 27 million sq mi (70 million sq km) and has an average depth of about 12,760 ft (3,890 m).
• Southern Ocean – covers an area of about 8.5 million sq mi (22 million sq km) and has an average depth of roughly 10,800 to 11,000 ft (3,300 to 3,400 m)
• Arctic Ocean – covers an area of about 5.4 million sq mi (14 million sq km) and has an average depth of about 3,950 ft (1,205 m).

World’s Major Seas

Compared to oceans, seas are far smaller but vary widely in size. There are about 100 seas worldwide. Here is a list of the world’s major seas:

• Philippine Sea – covers an area of about 2 million sq mi (5.2 million sq km) and has an average depth of about 13,773 ft (4,200 m).
• Sargasso Sea – covers an area of about 2 million sq mi (5.2 million sq km) and has an average depth of about 4,921 ft (1,500 m).
• Coral Sea – covers an area of about 1,850,000 sq mi (4.79 million sq km) and has an average depth of about 7,874 ft (2,400 m).
• Arabian Sea – covers an area of about 1,491,000 sq mi (3.86 million sq km) and has an average depth of about 8,970 ft (2,734 m).
• South China Sea – covers an area of about 1,423,000 sq mi (3.7 million sq km) and has an average depth of about 5,419 ft (1,652 m).
• Caribbean Sea – covers an area of about 1,063,000 sq mi (2,754,000 sq km) and has an average depth of about 8,685 ft (2,648 m).
• Mediterranean Sea – covers an area of about 970,000 sq mi (2.5 million sq km) and has an average depth of about 4,900 ft (1,500 m).
• Tasman Sea – covers an area of about 900,000 sq mi (2.3 million sq km) and has an average depth of about 17,000 ft (5,180 m).
• Bering Sea – covers an area of about 876,000 sq mi (2.27 million sq km) and has an average depth of about 5,075 ft (1,547 m).
• Sea of Okhotsk – covers an area of about 611,000 sq mi (1.58 million sq km) and has an average depth of about 2,818 ft (859 m).

Certain bodies of water blur the line between oceans and seas. For instance, the Sargasso Sea in the North Atlantic is not enclosed by land at all. However, it is considered a sea because it’s recognized as a distinct region, with ocean currents defining its perimeter.