All oceans are seas, but not all seas are oceans. 每个oceans是seas,但是些seas是oceans. All seas are bigger than lakes, but most are smaller than oceans. 每个 sea比湖大, 但是大多数的 sea比 ocean小. There are only five oceans on Earth: the Atlantic Ocean, the Pacific, the Arctic, the Antarctic and the Indian Ocean. 上地球上只一有五个oceans: the Atlantic Ocean, the Pacific Ocean, the Arctic Ocean, the Antarctic Ocean and the Indian Ocean. Collectively, these oceans and any sea that connects to them can be called 'the Ocean'. 每个在 ocean的水, 而且每个连接一个 ocean的sea,都是一个大 ocean. 它叫 'the ocean'.

ocean是特指（五大洋的）大洋，比如the Atlantic Ocean 大西洋，或者（总称) 海洋； sea单纯指海洋、海滨，比如the north sea 北海。

Often sea and ocean are used interchangeably in English: To "move overseas", "go to sea"', "roam the seven seas" may all refer to oceans rather than seas. Many native speakers incorrectly think "the seven seas" refer to the oceans of the world.

sea = 海 ocean = 洋