atrium (pl. atria).
1. Small court or principal room in a Roman house, called cavaedium or cavum aedium, usually surrounded by a roofed area, supported on columns, disposed with the compluvium or roofless opening in the centre. Rainwater was channelled into an impluvium, cistern, pool, or tank set under the opening in the floor. Types of domestic atrium include:atrium Corinthium: with more than four columns (i.e. a peristyle) supporting the edge of the compluvium;atrium displuvatium: with the roof sloping away from the compluvium, the rain being carried away by means of gutters and pipes;atrium testudinatum: with no compluvium, but crowned with an arched vault (testudo);atrium tetrastylum: with four columns, one at each corner supporting the compluvium;atrium Tuscanicum: insignificant and astylar with the roof carried on two beams with two short beams or trimmers.

2. Open court surrounded by a roofed arcaded or colonnaded walk, laid out before the west end of an Early Christian, Byzantine or medieval church, sometimes planted with trees, and often with a fountain in the centre. An interesting survival is the atrium of Sant'Ambrogio, Milan (c.1140). In this sense, the atrium was the forerunner of the cloister.

3. Top-lit volume surrounded by several storeys, e.g. Portman's Hyatt Regency Hotel, Atlanta, GA (1967–71).

Bibliography

Blaser (1985);
D. S. Robertson (1945);
Saxon (1994)

a·tri·um / ˈātrēəm/ • n. (pl. a·tri·a / ˈātrēə/ or a·tri·ums) 1. Archit. an open-roofed entrance hall or central court in an ancient Roman house. ∎  a central hall or court in a modern building, with rooms or galleries opening off it, often glass-covered.2. Anat. each of the two upper cavities of the heart from which blood is passed to the ventricles. The right atrium receives deoxygenated blood from the veins of the body; the left atrium receives oxygenated blood from the pulmonary vein. Also called auricle.DERIVATIVES: a·tri·al / ˈātrēəl/ adj.

atrium
1. In Urochordata and Cephalochordata, a chamber surrounding the pharyngeal region which receives water from the pharyngeal clefts and opens to the exterior through an atrial opening (sometimes called an atriopore).

2. See AURICLE.

atrium
1. (or auricle) A chamber of the heart that receives blood from the veins and forces it by powerful muscular contraction into the ventricle(s). Fish have a single atrium but all other vertebrates have two.

2. Any of various cavities or chambers in animals, such as the chamber surrounding the gill slits of the lancelet and other invertebrate chordates.

atrium (ay-tri-ŭm) n. (pl. atria)
1. either of the two upper chambers of the heart. The left atrium receives arterial blood from the lungs via the pulmonary artery; the right atrium receives venous blood from the venae cavae. See also auricle.

2. any of various anatomical chambers into which one or more cavities open.
—atrial adj.