atrium

‖n.; pl. Atria [ L., the fore court of a Roman house. ] 1. (Arch.) (a) A square hall lighted from above, into which rooms open at one or more levels. (b) An open court with a porch or gallery around three or more sides; especially at the entrance of a basilica or other church. The name was extended in the Middle Ages to the open churchyard or cemetery. [ 1913 Webster ]

2. (Anat.) The main part of either auricle of the heart as distinct from the auricular appendix. Also, the whole articular portion of the heart. [ 1913 Webster ]

3. (Zool.) A cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. See Ascidioidea. [ 1913 Webster ]

4. (Anat.) 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. [ Webster 1913 Suppl. ]