1. A court; the central hall or court of a Roman house.
2. A covered court or portico in front of the principal doors of churches, etc.
3. In a modern house, a central hall or glassed-in court that may be used as a sitting-room, having rooms opening off it, sometimes at more than one level.
4. In a public building, usually a skylit central court rising through several stories and surrounded by galleries at each level with rooms (shops, offices, etc.) opening off them.
5. In physiology, either of the two upper cavities (left and right atrium) of the heart into which the veins pour the blood.
6. In zoology, a large cavity into which the intestine opens.
7. In medicine, a chamber or cavity to which are connected several chambers or passageways: "On both sides of the heart, the atrium is the chamber that leads to the ventricle which is one of the two lower parts of the heart that moves blood to the rest of the body."