any chamber that is connected to other chambers or passageways (especially one of the two upper chambers of the heart)
"During his anatomy exam, the student struggled to visualize how blood flows from the right atrium into the right ventricle."
the central area in a building; open to the sky
"We sat on a bench in the atrium, enjoying the sunlight filtering down from the glass ceiling above us."
A central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
"The tour group paused under the glass dome of the atrium to admire how sunlight flooded the center of the historic building."