1) The formal entrance hall of an ancient Roman house, open to the sky at the center and often featuring a pool for the collection of rain water (impluvium).

Atrium, House of the Silver Wedding, Pompeii, Italy. 40-30 BCE.

2)  In a basilica church, a colonnaded courtyard preceding the entrance.

Old St. Peter’s Basilica, Rome. Reconstructed plan.

3) In contemporary architecture, an inner courtyard located near the entrance to a building, often lit with skylights.

Norman Foster. Atrium, Hearst Tower, New York. 2006.