What is a mainframe?

Mainframes (often colloquially referred to as big iron) are large and "expensive" computers used mainly by government institutions and large companies for mission critical applications, typically bulk data processing such as censuses, industry/consumer statistics, ERP, and financial transaction processing.

The term originated during 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").

Description
Modern mainframe computers have abilities not so much defined by their performance capabilities as by their high-quality internal engineering and resulting proven reliability, high-quality technical support, top-notch security, and strict backward compatibility for older software. These machines can and do run successfully for years without 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.

The robustness and dependability of these systems has been one of the main reasons for the longevity of this class of computers, being used in applications where down-time would be catastrophic. The term Reliability, Availability and Serviceability, or RAS, has become a marketing term used to denote this robustness. This robustness is often the argument used against replacing mainframes with other types of computers.

Mainframes often support thousands of simultaneous users who gain access through "dumb" terminals or terminal emulation. Early mainframes either supported this timesharing mode or operated in batch mode where users had no direct access to the computing service; it solely provided back office functions. At this time mainframes were so called because of their very substantial size and requirements for specialized HVAC and electrical power. Nowadays mainframes support access via any user interface, including the Web. (An IBM mainframe was the first Web server anywhere outside Europe.) Blade servers and rack servers, rather than mainframes, are now increasingly requiring exotic cooling technologies and take up far more data center floor space.