The Da Vinci Code vocabulary

The portico in front of ancient churches; sometimes, the atrium or outer court surrounded by ambulatories; -- used, generally, for any vestibule, lobby, or outer porch, leading to the nave of a church.

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an architectural element typical of early Christian and Byzantine basilicas and churches consisting of the entrance or lobby area, located at the west end of the nave, opposite the church's main altar. Traditionally [it] was a part of the church building, but was not considered part of the church proper.