Mainframe computer

Mainframes are large and "expensive" computers used mainly by government institutions and large companies for mission critical applications, characteristically bulk data processing such as censuses, industry/consumer statistics, ERP, and financial transaction processing.
The term originated through the early 1970s with the introduction of smaller, less complex computers such as the DEC PDP-8 and PDP-11 series, which became known as minicomputers or just minis. The industry/users then coined the term "mainframe" to describe larger, earlier types (previously known simply as "computers").
Modern mainframe computers contain abilities not so much defined by their performance capabilities as by their high-quality internal engineering and resulting proven reliability, expensive but high-quality technical support, top-notch security, and strict backward compatibility for older software. These machines can and do run successfully for years with no interruption, with repairs taking place whilst they continue to run. Mainframe vendors offer such services as off-site redundancy — if a machine does break down, the vendor offers the option to run customers' applications on their own machines (often without users even noticing the change) whilst repairs go on.