Atrium

Etymology
From ātrium (entry hall)

Noun

 * 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 square hall lit by daylight from above, into which rooms open at one or more levels.
 * 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.

Synonyms

 * cavaedium