Source : Webster's Revised Unabridged Dictionary (1913)

Narthex \Nar"thex\, n. [L., giant fennel, Gr. ?.]
   1. (Bot.) A tall umbelliferous plant ({Ferula communis}). See
      {Giant fennel}, under {Fennel}.

   2. (Arch.) 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.

Source : WordNet®

narthex
     n 1: portico at the west end of an early Christian basilica or
          church
     2: a vestibule leading to the nave of a church