Prothyrum

A prothyrum (Romanization of Greek πρόθυρον próthyron 'in front of the door'), in classical and medieval architecture, is a small porch, vestibule, or covered space immediately in front of the main doorway of a building. In architecture of the Greco-Roman world it was the transitional, often columned, space before the entrance proper. In the Late Antique and Byzantine periods, it could be a forecourt or portico preceding the narthex of a church or the main gateway of a monastic or palatial ensemble.