ocean (n.)

c. 1300, occean, "the vast body of water on the surface of the globe," from Old French occean "ocean" (12c., Modern French océan), from Latin oceanus, from Greek ōkeanos, the great river or sea surrounding the disk of the Earth (as opposed to the Mediterranean), a word of unknown origin; Beekes suggests it is Pre-Greek. Personified as Oceanus, son of Uranus and Gaia and husband of Tethys.

In early times, when the only known land masses were Eurasia and Africa, the ocean was an endless river that flowed around them. Until c. 1650, commonly ocean sea, translating Latin mare oceanum. Application to individual bodies of water began 14c. (occean Atlantyke, 1387); five of them are usually reckoned, but this is arbitrary. The English word also occasionally was applied to smaller subdivisions, such as German Ocean "North Sea."