Real Dictionary

auditorium
countable noun: An auditorium is a large room, hall, or building which is used for events such as meetings and concerts.
countable noun: An auditorium is the part of a theatre or concert hall where the audience sits.

auditorium in American English
a room for the gathering of an audience, as in a school, library, etc.
noun: the space set apart for the audience in a theater, school, or other public building

auditorium in British English
noun: the area of a concert hall, theatre, school, etc, in which the audience sits