Atrium

pl. Atria

1. A square hall lighted from above, into which rooms open at one or more levels. (Arch.)
2. The main part of either auricle of the heart as distinct from the auricular appendix. Also, the whole articular portion of the heart. (Anat.)
3. A cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. See Ascidioidea. (Zool.)
4. A cavity, entrance, or passage; as, the atrium, or atrial cavity, in the body wall of the amphioxus; an atrium of the infundibula of the lungs, etc. (Anat.)