Yes, I missed a week last week, but this incredibly popular feature is back due to overwhelming demand by the readership. (Not really) ANYway:
IMPLUVIUM. The basin or water cistern, usually rectangular, in the centre of an ATRIUM of a Roman house to receive the rain-water from the surrounding roofs. The term is also used, loosely, for the uncovered space in the atrium as well as the water cistern.
And to complete the definition, the companion word--ATRIUM. 1. In Roman domestic architecture, an inner court open to the sky and surrounded by the roof. 2. In Early Christian and medieval architecture, an open court in front of a church; usually a colonnaded quadrangle.
(Both definitions, as always, from the Penguin Dictionary of Architecture, Third Edition)