Webster's Dictionary 
(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.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
In ancient churches, between the first porch, called the propylaeum, or vestibulum magnum, and the church itself, was a large area or square plot of ground, which the Latins called atrium or impluvium, because it was a court open to the air without any covering. It was surrounded by cloisters. In this place stood the first class of penitents, according to Eusebius, who says it was the mansion of those who were not allowed to enter farther into the church. They generally stood in this porch to beg the prayers of the faithful. — Binghamn, Orig. Eccl. bk. 8, ch. 3, § 5.