› [C, U] a large area of salt water: I'd like to live by the sea. We went swimming in the sea. It was our third day at sea (= travelling on the sea). It's cheaper to send parcels by sea (= on a ship).
Sea › a particular area of salt water: the North Sea the Black Sea
a sea of sth › a large number of something: He looked across the room and saw a sea of faces.