Lacus Aestatis consists of two small areas of lunar mare located near the western limb. The lake has a combined surface area in the order of 1,000 km2. The northwestern part of this feature is located to the east-southeast of the crater Rocca, and is contained within the rim of the satellite crater Rocca A. The other part lies to the southeast of the first, and forms an elongated, irregular patch that extends generally in a north-south direction. (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.