A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, the term refers to a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean. It is also used sometimes to describe a large saline lake that lacks a natural outlet, such as the Caspian Sea.
The phrase "Seven Seas" (as in the idiom "sail the Seven Seas") can refer either to a particular set of seven seas or to a great expanse of water in general. People from different time periods have meant different things by the phrase "Seven Seas".
went down to Basra with a group of merchants and companions, and we set sail in a ship upon the sea, and at first I was seasick because of the waves and the motion of the vessel, but soon I came to myself and we went about among the islands, buying and selling.”