1. The part of a theater designed to accommodate an audience.
2. A large room to accommodate an audience in a building; such as, a school or theater.
3. A large building for public meetings or performances.
4. Etymology: from Latin auditorium, "lecture room"; literally, "a place where something is heard"; neuter of auditorius, "of" or "for hearing"; from auditor, "a listener"; from audire, "to hear".

The auditorium can also describe an entire theater, and has been in use as a word since the 18th century, although there were other words with the same meaning before that.