All 3 Uses

atrium
in
The Lost Hero
(Auto-generated)

The upper floors made a ring of balconies around a huge central atrium, so they could see all the way down to the ground floor.†
Chpt 26
In the middle of the atrium a fountain sprayed water twenty feet into the air, changing color from red to yellow to blue.†
Chpt 26 *
Festus flew halfway up the atrium, then hurled the sun dragons into the pits they'd come from.†
Chpt 28

Definitions:

(1) (atrium as in: atrium of the building)
a central open space in a building -- often a large skylit hall near the entrance

In ancient Roman houses and early Christian churches, an atrium was an open courtyard. In many modern buildings, it is an indoor space that may include plants, water features, and balconies or walkways to other rooms.
(2) (atrium as in: right atrium of the heart)
chamber from which blood enters the heart
(3) (meaning too rare to warrant focus)
Much less commonly, atrium can refer to anatomical structures other than the heart.