singular/uncountable the large area of salt water that covers most of the surface of the EarthTim went swimming in the sea.
He had a room overlooking the sea.
We’re renting a house by the sea (=close to the sea).
into the sea: I ran across the beach and into the sea.
at sea (=in a boat or ship on the sea): Some older tankers need maintenance while at sea.
by sea (=in a ship across the sea): Most of his belongings will be transported by sea.
out to sea (=out onto the sea): She rowed the boat out to sea.
lost at sea (=died in the sea): A service was held to commemorate those lost at sea.
put to sea (=start a journey in a ship): The cruiser put to sea yesterday from the port of Murmansk.
the open sea (=a part of the sea far from land): The yacht was on the open sea in light winds.