What is Mainframe?

Definition:                  A mainframe (also known as "big iron") is a high-performance computer used for large-scale computing...

Definition:
                 A mainframe (also known as "big iron") is a high-performance computer used for large-scale computing purposes that require greater availability and security than a smaller-scale machine can offer. Historically, mainframes have been associated with centralized rather than distributed computing, although that distinction is blurring as smaller computers become more powerful and mainframes become more multi-purpose. Today, IBM emphasizes that their mainframes can be used to serve distributed users and smaller servers in a computing network.
The mainframe is sometimes referred to as a "dinosaur" not only because of its size but because of reports, going back many years, that it's becoming extinct. In 1991 Stewart Alsop, the editor of InfoWorld, predicted that the last mainframe would be retired by 1996. However, in February 2008 IBM released a new mainframe, the z10. Steve Lohr wrote about the mainframe as "the classic survivor technology" in The New York Times ("Why old technologies are still kicking"):

I.B.M. overhauled the insides of the mainframe, using low-cost microprocessors as the computing engine. The company invested and updated the mainframe software, so that banks, corporations and government agencies could still rely on the mainframe as the rock-solid reliable and secure computer for vital transactions and data, while allowing it to take on new chores like running Web-based programs.
The original mainframes were housed in room-sized metal frames, which is probably where the name derives from. In the past, a typical mainframe might have occupied 2,000 - 10,000 square feet. Newer mainframes are about the same size as a large refrigerator.