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

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "auditorium":
   Elizabethan theater, Globe Theatre, Greek theater, agora,
   amphitheater, arena, arena theater, assembly hall, athletic field,
   background, balcony, bear garden, bowl, box, box seat, boxing ring,
   bull ring, cabaret, campus, canvas, chapel, circle theater, circus,
   classroom, club, cockpit, coliseum, colosseum, concert hall,
   convention hall, course, dance hall, dress circle, exhibition hall,
   fauteuil, field, floor, forum, gallery, ground, gym, gymnasium,
   hall, hippodrome, house, lecture hall, lecture room, lists,
   little theater, locale, loge, marketplace, mat, meetinghouse,
   milieu, music hall, nigger heaven, night spot, nightclub,
   open forum, opera, opera house, orchestra, orchestra circle,
   outdoor theater, palaestra, parade ground, paradise, parquet,
   parquet circle, parterre, peanut gallery, pit, place, platform,
   playhouse, precinct, prize ring, proscenium boxes, public square,
   purlieu, range, recitation room, ring, scene, scene of action,
   scenery, schoolroom, setting, showboat, site, sphere,
   squared circle, stadium, stage, stage set, stage setting, stall,
   standing room, terrain, theater, theater-in-the-round,
   theatre stall, theatron, tilting ground, tiltyard, walk,
   wrestling ring