The entrance porch or vestibule at the western end of many early Christian, Byzantine, Romanesque, and later churches, preceding the nave.
A narthex is an entrance porch or vestibule associated especially with early Christian and Byzantine churches and, in later usage, with the western entrance zone of a church before the nave. It forms a transitional architectural space between the exterior and the principal interior of the church.
For the study of sculpture, the narthex is important because entrances and transitional spaces frequently carry portals, capitals, reliefs, tomb sculpture, devotional images, and other programs of architectural carving.
Etymology
From Greek narthēx, originally “giant fennel” or a hollow case; the word was later applied to the entrance vestibule of a church.
Sculptor Notes
The narthex matters to the sculptor less as an isolated room than as a setting for architectural sculpture. It is part of the ceremonial sequence by which carved portals, figures, reliefs, and ornament prepare the viewer to enter the sacred interior.
References
https://www.pierres-info.fr/glossaire/index.html; https://www.getty.edu/vow/AATFullDisplay?subjectid=300004624