Merriam-Webster Collegiate Dictionary - atrium

 noun  (plural atria; also ~s)  Etymology: Latin  Date: 1577  1. the central room of a Roman house  2. plural usually ~s  a. a rectangular open patio around which a house is built  b. a many-storied court in a building (as a hotel) usually with a skylight  3. New Latin, from Latin an anatomical cavity or passage; especially the chamber or either of the chambers of the heart that receives blood from the veins and forces it into the ventricle or ventricles — see heart illustration  • atrial adjective