Mainframe, often called "Big Iron" (in United States) is a computer system designed in 50's, used mainly by universities and corporations for running its most critical applications (resource planning, census etc.). By the time first Mainframe was created the only alternative was minicomputers and calculators. They were huge, access through dumb terminals (thin client / static terminal – only communication in/out), no processing), and expensive, but their high stability and reliability allowes mainframes continously running for decades without any interruption. Mainframes are designed to handle transaction processing like inventory conrol, airline reservations, or banking.
Sharing the same mainframe by many users was the best way to cut costs of very expensive hardware. It is also described as a significant contribution to the development of the Internet and also a precursor of Cloud Computing – storing data and computing on a remote server via the Internet.