noun, plural atria [ey-tree-uh] , atriums.
Also called cavaedium. the main or central room of an ancient Roman house, open to the sky at the center and usually having a pool for the collection of rain water.
a courtyard, flanked or surrounded by porticoes, in front of an early or medieval Christian church. See diag. under basilica.
a skylit central court in a contemporary building or house.
Anatomy. either of the two upper chambers on each side of the heart that receive blood from the veins and in turn force it into the ventricles.

1570–80; < Latin (in anatomical sense < NL)

atrial, adjective
interatrial, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
atrium (ˈeɪtrɪəm, ˈɑː-)
n , pl atria
1.  the open main court of a Roman house
2.  a central often glass-roofed hall that extends through several storeys in a building, such as a shopping centre or hotel
3.  a court in front of an early Christian or medieval church, esp one flanked by colonnades
4.  anatomy a cavity or chamber in the body, esp the upper chamber of each half of the heart
[C17: from Latin; related to āter black, perhaps originally referring to the part of the house that was blackened by smoke from the hearth]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1570s, from L., "central court or main room of an ancient Roman house," sometimes said (on authority of Varro, "De Lingua Latina") to be an Etruscan word, but perhaps from PIE *ater- "fire," on notion of "place where smoke from the hearth escapes" (through a hole in the roof). Anatomical sense of "either
of the upper cavities of the heart" first recorded 1870. Meaning "skylit central court in a public building" first attested 1967. Related: Atrial (1869).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

atrium a·tri·um (ā'trē-əm)
n. pl. a·tri·ums or a·tri·a (ā'trē-ə)

  1. A chamber or cavity to which several chambers or passageways are connected.

  2. Either the right or the left upper chamber of the heart that receives blood from the veins and forces it into a ventricle.

  3. That part of the tympanic cavity that lies below the eardrum.

  4. A subdivision of the alveolar duct in the lung from which the alveolar sacs open.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
atrium   (ā'trē-əm)  Pronunciation Key 
Plural atria or atriums
A chamber of the heart that receives blood from the veins and forces it by muscular contraction into a ventricle. mammals, birds, reptiles, and amphibians have two atria; fish have one.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Picture the lobby atrium of a new, green building, one filled with leafy plants
  and trees.
We were shooting a commercial in the atrium of a building, where a small
  football field is.
Walk catwalks in central atrium to your room: floor-to-ceiling windows, sepia
  artwork, eco-friendly soap dispensers.
The building will have a cedar facade, nine peaks and a two-story atrium.
Images for atrium
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