Mainframe is a type of computer which is renowned for their large size as
well as the amount of storage they hold, processing power they have and their
high level of reliability. They are mainly used by large organisations got
mission-critical applications which require a high volume of data processing.
Almost all Mainframes have the ability to multiple operating systems.
Additionally, they can add or hot swap system capacity without disruption.
They have been designed to handle a very high volume input and output and emphasise quantity computing. This technique allows applications to run as if they were on physically different computers. A single Mainframe can even replace hundreds of smaller servers.
When working within the IT industry, especially within a large organisation, they are often used to replace multiple servers.