Atrium 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.