Lacus Mortis, latin for "Lake of Death", is a plain of basaltic lava flows in the northeastern part. It lies just to the south of the elongated Mare Frigoris, being separated by a slender arm of rugged ground. To the south is the Lacus Somniorum, separated from this mare by the joined craters Plana and Mason, and a strip of uneven surface. Located prominently just to the east of the midpoint of this feature is the crater Bürg. (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.