When discussing church architecture, the first area that is named is the atrium. While it may refer to a variety of places in a modern Catholic church, it was originally a very specific location.
A brief history of the atrium in a Catholic church
The atrium was initially the central court of Roman houses and was open to the air. Sometimes it would contain a hearth, where the fire would burn and bring warmth to the home.
Eventually the term was adopted by Catholics when designing their churches, denoting the outer courtyard of the church.
The Catholic Encyclopedia explains that it was, "An open place or court before a church. It consisted of a large quadrangle with colonnaded walks on its four sides forming a portico or cloister. It was situated between the porch or vestibule and the body of the church. In the center of the atrium was a fountain or well, where the worshipers washed their hands before entering the church."
Eusebius mentions this location in his Church History, explaining its spiritual symbolism.
Here he has placed symbols of sacred purifications, setting up fountains opposite the temple which furnish an abundance of water wherewith those who come within the sanctuary may purify themselves. This is the first halting-place of those who enter; and it furnishes at the same time a beautiful and splendid scene to every one, and to those who still need elementary instruction a fitting station.
While atriums gradually disappeared in Catholic Church architecture, a remnant of this place of ritual washing is retained in holy water fonts, which are used to remind us of our baptism and symbolize the spiritual purification we need.