(1):
(n.) 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.
(2):
(n.) A square hall lighted from above, into which rooms open at one or more levels.
(3):
(n.) The main part of either auricle of the heart as distinct from the auricular appendix. Also, the whole articular portion of the heart.
(4):
(n.) A cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. See Ascidioidea.
(5):
(n.) 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.