atrium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Architecture A rectangular court, as.
• noun A usually skylighted central area, often containing plants, in some modern buildings, especially of a public or commercial nature.
• noun The open area in the center of an ancient Roman house.
• noun The forecourt of a building, such as an early Christian church, enclosed on three or four sides with porticoes.
• noun Anatomy A body cavity or chamber, especially either of the upper chambers of the heart that receives blood from the veins and forces it into a ventricle.

from The Century Dictionary.

• noun In anc. Rom. arch., the entrance-hall, the most important and usually the most splendid apartment of the house.
• noun A hall or court resembling in arrangement an atrium proper, as at the entrance of some classical or early Christian public buildings, etc.
• noun [NL.] In anatomy, an auricle of the heart, or some equivalent venous cardiac cavity.
• noun [NL.] In zoology: The chamber or cavity of ascidians, communicating with the exterior, and with the cavity of the alimentary canal. See atrial, and cut under Tunicata.
• noun A membranous saccular diverticulum of the ear in fishes: as, the atrium sinus imparis, a membranous sac given off from the sinus auditorius impar of fishes, and connected in various ways with the air-bladder.

from the GNU version of the Collaborative International Dictionary of English.

• noun A square hall lighted from above, into which rooms open at one or more levels.
• noun An open court with a porch or gallery around three or more sides; especially at the entrance of a basilica or other church. The name was extended in the Middle Ages to the open churchyard or cemetery.
• noun (Anat.) The main part of either auricle of the heart as distinct from the auricular appendix. Also, the whole articular portion of the heart.
• noun (Zoöl.) A cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. See Ascidioidea.
• noun (Anat.) A cavity, entrance, or passage

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture A central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
• noun architecture A square hall lit by daylight from above, into which rooms open at one or more levels.
• noun anatomy Any enclosed body cavity or chamber
• noun anatomy An upper chamber of the heart that receives blood from the veins and forces it into a ventricle. In higher vertebrates, the right atrium receives blood from the superior vena cava and inferior vena cava, and the left atrium receives blood from the left and right pulmonary veins.
• noun anatomy A microscopic air sac within a pulmonary alveolus.
• noun palynology A cavity inside a porate aperture of a pollen grain formed by the separation of the sexine and nexine layers, widening toward the interior of the grain.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the central area in a building; open to the sky
• noun any chamber that is connected to other chambers or passageways (especially one of the two upper chambers of the heart)