Lacus Autumni is a region of lunar mare that lies near the western limb. Along this side of the lunar surface is a huge impact basin centered on the Mare Orientale. Two concentric mountain rings surround the Orientale mare, the inner ring being named Montes Rook and an outer ring called the Montes Cordillera. Lacus Autumni lies in the northeastern quadrant of the gap between these two mountain rings. (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.