atrium

noun7

1. •
   A central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
2. •
   A cavity, entrance, or passage.
3. •
   Any enclosed body cavity or chamber.
4. •
   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.
5. •
   A microscopic air sac within a pulmonary alveolus.
6. •
   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.

   • Nexine 0.5μ thick, separating from the sexine about 5μ from the pore and forming a deep, well-defined atrium. — 1965
7. •
   A square hall lit by daylight from above, into which rooms open at one or more levels.