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Architecture.

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.
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.
Contemporary Examples

He had reassembled the weapon in a bathroom and stepped out onto a fourth-floor walkway overlooking an atrium.
Inside the Washington Navy Yard’s Building 197 During Alexis’s Rampage Michael Daly September 18, 2013

The PBS broadcast will also be screened for free, beginning at 9:00 p.m., at the David Rubenstein atrium at Lincoln Center.
The New York Philharmonic and Credit Suisse Kick Off the 2010-11 Season with a Free Dress Rehearsal Daily Beast Promotions September 21, 2010

He was wounded as he and a number of comrades exchanged fire with Alexis, by one account across the atrium.
Inside the Washington Navy Yard’s Building 197 During Alexis’s Rampage Michael Daly September 18, 2013

Historical Examples

He is not known to be represented anywhere in art, save in the almost obliterated frescoes in the atrium of this church.
Walks in Rome Augustus J.C. Hare

In the house of Vettius the two money chests were found in the atrium.
Buried Cities: Pompeii, Olympia, Mycenae Jennie Hall

The Dipnoans37 show an important advance on the conus as in atrium and ventricle.
Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 3 Various

This basin was in the center of the atrium, the most important room in the house.
Buried Cities: Pompeii, Olympia, Mycenae Jennie Hall

The nearer doors belong to chambers which open into the atrium.
Museum of Antiquity L. W. Yaggy

Fauces, the passage from the atrium to the peristyle in a Roman house.
Architecture Thomas Roger Smith

He called the chief of the atrium then, and gave the needful orders.
Quo Vadis Henryk Sienkiewicz

noun (pl) atria (ˈeɪtrɪə; ˈɑː-)
the open main court of a Roman house
a central often glass-roofed hall that extends through several storeys in a building, such as a shopping centre or hotel
a court in front of an early Christian or medieval church, esp one flanked by colonnades
(anatomy) a cavity or chamber in the body, esp the upper chamber of each half of the heart
n.

1570s, from Latin atrium “central court or main room of an ancient Roman house, room which contains the hearth,” 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.

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

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

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

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

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

atrium
(ā’trē-əm)
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.