What is a mainframe computer?

From Wiki :

Mainframe computers (colloquially referred to as "big iron") are computers used primarily by large organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and transaction processing.

Mainframe computers are the huge ones business, schools , etc. Use in their place of business. These are the ones one usually sees on TV programs that fill up huge rooms

A very large and expensive computer capable of supporting hundreds, or even thousands, of users simultaneously.

A mainframe is an ultra high-performance computer made for high-volume, processor-intensive computing. They are typically used by large businesses and for scientific purposes. You probably won't find a mainframe in any household. In the hierarchy of computers, mainframes are right below supercomputers, the most powerful computers in the world.