ATRIUM, originally the central room of a Roman house in which was placed the hearth. As this room had a hole in the roof to let out the smoke the atrium was in essence a small court, and when with the developing complexity of the Roman house the kitchen and hearth were removed to other positions the atrium remained as a court serving as a formal reception room and as the official centre of family life. By the end of the republic the addition of one or more colonnaded courts in the larger houses removed from the atrium the last vestiges of family life and in the empire it became practically the office of the owner of the house. The atrium might be either with or without columns; it had, universally, a marble basin which was known as the im pluvium. This was situated in the centre under the opening in the roof called the compluvium.
The term atrium is used in a generic sense (like the English "hall") as in the Atrium Vestae, the house of the vestal virgins. The word was later used for any open court, especially that around a temple or in front of a Christian church as in San Clemente at Rome and San Ambrogio at Milan. Occasionally the word is in correctly used for narthex (q.v.) . (T. F. H.)