Source : Webster's Revised Unabridged Dictionary (1913)

Atonement \A*tone"ment\, n.

   {Day of Atonement} (Jewish Antiq.), the only fast day of the
      Mosaic ritual, celebrated on the tenth day of the seventh
      month (Tisri), according to the rites described in
      Leviticus xvi. Atrium \A"tri*um\, n. (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.

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.

   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. (Zo["o]l.) A cavity in ascidians into which the intestine
      and generative ducts open, and which also receives the
      water from the gills. See {Ascidioidea}.

Source : WordNet®

atrium
     n 1: any chamber that is connected to other chambers or
          passageways (especially one of the two upper chambers of
          the heart)
     2: the central area in a building; open to the sky
     [also: {atria} (pl)]