Lacus Somniorum is a plain located in the northeastern part 's near side. (It is the largest of the lunar features designated Lacus.) The name is Latin for Lake of Dreams, a title given to this feature by Riccioli. Lacus Somniorum is an irregular feature with complex, somewhat ill-defined borders. (wikipedia entry)
A lacus is a "lake" or small plain. On the Moon they are named after latin terms describing weather and other abstract concepts.