Compluvium

(from compluere, to flow together, e.g. in reference to the rain being collected and  drained), in architecture, the Latin term used for the open space left in the roof of the atrium of a Roman house providing light  to the yard and the rooms around it, as well as for collecting water.

In reference to cisterns it signified an inclined collecting plain from where water ran or fell into the collector called impluvium.