Compluvium

Compluvium: (from compluere, to flow together, i.e. in reference to the rain being collected and falling through), in architecture, the Latin term for the open space left in the roof of the atrium (=yard) of a Roman house for lighting it and the rooms round, as well as for collecting water.

Referred to cisterns it was an inclined collecting plain from where water went or fell down in the collector called impluvium.