A1[ C or U ]
the salty water that covers a large part of the surface of the earth, or a large area of salty water, smaller than an ocean, that is partly or completely surrounded by land:
the Mediterranean Sea
We went swimming in the sea.
The sea was calm/smooth/choppy/rough when we crossed the Channel.
The refugees were at sea (= in a boat on the sea a long way from land) for 40 days before reaching land.
When we moved to the US, we sent our things by sea (= in a ship).
We spent a week by the sea (= on the coast) this year.
Soon we had left the marina and were heading towards the open sea (= the part of the sea a long way from land).
[ C ]
one of the large, flat areas on the moon that in the past were thought to be seas