Mare Tranquillitatis
Mare Tranquillitatis is a large area on the visible side of the Moon.  It is also called the Sea of Tranquillity.  It is called a sea because ancient astronomers looked at the Moon and thought they saw seas and oceans on the Moon.  Now we know that they are not seas.  They are darkened areas on the Moon's surface.  They may have been made by ancient volcanoes by things such as leaks and eruptions.  Mare Tranquillitatis is just one of the 22 "Seas" and "Oceans" on the Moon.