What is a Mainframe?

Mainframe computers, colloquially referred to as "big iron" are computers used primarily by corporate and governmental 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.
Source: Wikipedia

Having read John Campbell's "What is a Mainframe", and having been asked this question myself many times I would like to propose a more illuminating definition. First, however, some very brief biographical info. I first became interested in computing machines as a teenager. In those days the 2nd generation was rapidly drawing to a close and System/360 was about to change the computing landscape. My first programming experience was in high school, where my class had access to a very fast IBM 7094-II (and before you ask, no, my high school did not have its own 7094; we were allowed limited use of one of MIT's systems). people In college I majored in math, primarily because computer science as a major was still about 4 years in the future. Nevertheless, my first love has always been computing machines, and I have invested a lifetime of study and labor in this industry. I have worked with all platforms except vector processing based supercomputers. My favorite has always been, and remains to this day, the mainframe.

One might suppose that it would be easy to define a mainframe, but such is not the case. Some definitions are so broad that they include all computing platforms. Others seek to concentrate on some particular aspect of mainframe computing (such as the operating systems which run on a mainframe) and declare that a mainframe is that which runs or supports this computing aspect. This latter definition suffers from two problems: 1) it is completely unenlightening; and 2) it is misleading. For example, the FLEX/ES simulator allows one to run OS/390, VM, and VSE/ESA on a fast Intel processor. Yet most people who have worked with both classes of machine would intuitively consider the Intel PC to be the opposite of a mainframe. (Read more)

IBM mainframes dominate the mainframe market at well over 90% market share. Unisys manufactures ClearPath Libra mainframes, based on earlier Burroughs products and ClearPath Dorado mainframes based on Sperry Univac OS 1100 product lines. In 2002, Hitachi co-developed the zSeries z800 with IBM to share expenses, but subsequently the two companies have not collaborated on new Hitachi models. Hewlett-Packard sells its unique NonStop systems, which it acquired with Tandem Computers and which some analysts classify as mainframes. Groupe Bull's DPS, Fujitsu (formerly Siemens) BS2000, and Fujitsu-ICL VME mainframes are still available in Europe. Fujitsu, Hitachi, and NEC (the "JCMs") still maintain mainframe hardware businesses in the Japanese market.
Source: Wikipedia

The System z10™ family of servers is well positioned to participate in this new dynamic model. It delivers many innovative technologies for ﬂ exible enterprise computing and includes proven leadership capabilities for security, availability, scalability, virtualization, and management. As environmental concerns raise the focus on energy consumption, the System z10 is designed to reduce energy usage and save ﬂoor space when consolidating workloads from distributed servers. The System z10 specialty engines continue to help users expand the use of the mainframe for a broad set of applications, while helping to lower the cost of ownership.
Source: IBM

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