1. Open vestibule in an Antique Classical temple, to the rear of the cella or naos, also called epinaos or opisthodomus, corresponding to the pronaos, really part of a back portico.
2. Inferior, subsidiary, or rear door in an Antique Roman house.
"posticum." A Dictionary of Architecture and Landscape Architecture. . Encyclopedia.com. (August 21, 2018). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/posticum
"posticum." A Dictionary of Architecture and Landscape Architecture. . Retrieved August 21, 2018 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/posticum