A narthex is an entrance porch or vestibule at the west end of a church, physically and often visually distinct from the nave it leads into, forming a transitional space between the outside world and the main body of the church. It can take several architectural forms: an open, unroofed portico running along the facade, a fully enclosed covered porch, or, in some Byzantine and Eastern churches, an inner narthex (esonarthex) placed directly against the nave wall combined with an outer narthex (exonarthex) further from the building, creating two successive stages of transition rather than one.
Because the narthex sits outside or at the very edge of the nave proper, it has historically functioned as a space for those not yet fully admitted to participate in the central liturgy, and in many church traditions it remains today a practical gathering space for greeting, for those arriving late, or for activities, such as receiving newcomers or holding a brief service, that do not require the full liturgical setting of the nave itself. Architecturally the narthex also frequently serves as the transition point where a visitor's eye first adjusts from the outside light and open space of the exterior to the interior scale, darkness and orientation of the church proper.