The Collaborative International Dictionary of English v.0.48:

Atrium \A"tri*um\, 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.]