Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

auditorium
     n : the area of a theater or concert hall where the audience
         sits
     [also: {auditoria} (pl)]