›[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 Seathe Black Sea
a sea of sth›a large number of something: He looked across the room and saw a sea of faces.