In Byzantine architecture, the outer courtyard before a church’s narthex is called an atrium.
The atrium was an open court, often surrounded by covered walkways, placed before the church entrance. Worshippers approaching a major basilica could pass through this space before reaching the narthex, the enclosed vestibule immediately inside the main doors.
Large Byzantine churches commonly used a sequence of spaces: an atrium outside, a narthex within the entrance zone, the nave for the congregation, and the sanctuary at the eastern end. This arrangement reflected both practical circulation and distinctions between areas of worship.
The atrium should not be confused with the exonarthex. An exonarthex is an outer vestibule or porch, while an atrium is fundamentally an open courtyard. The term also appears in descriptions of Roman houses and early Christian basilicas, so its exact architectural setting matters.