What is the "Ocean Sea"?

I've come across the term "Ocean Sea" and wonder what it means, especially compared to simpler terms like "ocean" or "sea"? Is this a recognized term in English, either current or historical?

Solution 1:

The Romans recognized two major seas: the mare mediterraneum or “sea in the middle of the earth”, and the mare oceanum, or “Sea of Oceanus” — Okeanos being the Greek name for and personification of the great “river” believed to encircle the earth.

In English, mare oceanus was translated Ocean Sea, as opposed to the Mediterranean Sea, and it was only about 1650 (according to the OED, volume 7, online here) that ocean started to appear alone as a noun designating the vast outer water, so much greater than mere local seas.