The Collaborative International Dictionary of English v.0.48:

Auditorium \Au`di*to"ri*um\, n. [L. See Auditory, n.]
   1. The part of a church, theater, or other public building,
      assigned to the audience.
      [1913 Webster]

   Note: In ancient churches the auditorium was the nave, where
         hearers stood to be instructed; in monasteries it was
         an apartment for the reception of strangers.
         [1913 Webster]

   2. a building, or a room within a building, containing a
      large open space for public gatherings, and often having a
      raised stage where speakers or presentations may be easily
      viewed by the audience. The students assembled to hear the
      president speak in the auditorium.
      [PJC]

WordNet (r) 3.0 (2006):

auditorium
    n 1: the area of a theater or concert hall where the audience
         sits