In architecture, the term xystum refers to a wall, promenade, alley, or open path. It can also refer to an atrium, ambulacrum, or parvis in front of a basilica. The term should not be confused with the ancient Greek architectural term xystus, meaning the covered portico of a gymnasium.
Book: Curl , James Stevens . A Dictionary of Architecture and Landscape Architecture . 2006 . registration . Paperback . Second . Oxford University Press . 0-19-860678-8 . 880 .