Definitions
1.NounA central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
2.NounA square hall lit by daylight from above, into which rooms open at one or more levels.
3.NounA cavity, entrance, or passage."an atrium of the infundibula of the lungs"
4.NounAny enclosed body cavity or chamber.
5.NounAn 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.
6.NounA microscopic air sac within a pulmonary alveolus.
7.NounA 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.