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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

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: Webster's Revised Unabridged Dictionary (1913)