Mainframe

Mainframe computers (colloquially referred to as "big iron") are computers used primarily by large organizations for critical applications, bulk data processing, such as census, industry and consumer statistics, enterprise resource planning, and transaction processing.

The term originally referred to the large cabinets called "main frames" that housed the central processing unit and main memory of early computers. Later, the term was used to distinguish high-end commercial machines from less powerful units. Most large-scale computer system architectures were established in the 1960s, but continue to evolve.

Modern mainframe design is generally less defined by single-task computational speed (typically defined as MIPS rate or FLOPS in the case of floating point calculations), and more by:

Their high stability and reliability enable these machines to run uninterrupted for decades.

Software upgrades usually require setting up the operating system or portions thereof, and are non-disruptive only when using virtualizing facilities such as IBM's z/OS and Parallel Sysplex, or Unisys's XPCL, which support workload sharing so that one system can take over another's application while it is being refreshed. Mainframes are defined by high availability, one of the main reasons for their longevity, since they are typically used in applications where downtime would be costly or catastrophic. The term reliability, availability and serviceability (RAS) is a defining characteristic of mainframe computers. Proper planning and implementation is required to exploit these features, and if improperly implemented, may serve to inhibit the benefits provided. In addition, mainframes are more secure than other computer types: the NIST vulnerabilities database, US-CERT, rates traditional mainframes such as IBM zSeries, Unisys Dorado and Unisys Libra as among the most secure with vulnerabilities in the low single digits as compared with thousands for Windows, Unix, and Linux.

...
Wikipedia