File:Honeywell-Bull DPS 7 Mainframe BWW March 1990.jpg
A 1990 Honeywell-Bull DPS 7 mainframe
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.