The Collaborative International Dictionary of English v.0.48:

Narthex \Nar"thex\, n. [L., giant fennel, Gr. na`rqhx.]
   1. (Bot.) A tall umbelliferous plant (Ferula communis). See
      Giant fennel, under Fennel.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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