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Mainframes (often colloquially referred to as Big Iron) are computers used mainly by government institutions and large companies for mission critical applications, typically bulk data processing such as census, industry/consumer statistics, ERP, and financial transaction processing.
The term originated during the early years of computing and referred to the large mechanical assembly that held the central processor and input/output complex. Later the term was used to distinguish high-end commercial machines from less powerful units which were often contained in smaller packages. Today, this term almost exclusively refers to IBM zSeries mainframes - descendants of the System/360.