Atrium
in ancient times, the hall or principal room in an ancient Roman house. It communicated with the street by the vestibule and the front door.
There was in the center of its ceiling a large aperture, called compluvium, designed to admit light. Beneath it there was scooped out in the pavement a cistern called impluvium. In a large house, rooms opened into the atrium from all sides, and were lighted from it.
In medieval times, till the 12th century, a covered court. After the 12th century, the churchyard.