noun

• (architecture) A central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
• (architecture) A square hall lit by daylight from above, into which rooms open at one or more levels.
• (anatomy) A cavity, entrance, or passage.
• (biology) Any enclosed body cavity or chamber.
• (anatomy) 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.
• (anatomy) A microscopic air sac within a pulmonary alveolus.
• (palynology) 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.