1) The formal entrance hall of an ancient Roman house, open to the sky at the center and usually featuring a pool for the collection of rain water.
Atrium, House of the Silver Wedding, Pompeii.
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.