The Collaborative International Dictionary of English v.0.48:
Atrium \A"tri*um\, n.; pl. Atria. [L., the fore court of a
(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.
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.
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.
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.]