atrium

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, Medicine, Sciences A cavity, entrance, or passage.

Biology, Natural sciences Any enclosed body cavity or chamber.

Anatomy, Medicine, Sciences 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, Medicine, Sciences A microscopic air sac within a pulmonary alveolus.

Biology, Chemistry, Microbiology, Natural sciences, Palynology, Physical sciences 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.