Noun

1. A square hall lit from above, into which rooms open at one or more levels
2. Any enclosed body cavity or chamber
3. 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.
4. A microscopic air sac within a pulmonary alveolus
5. 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