mainframe

Mainframes (often colloquially referred to as "big iron") are powerful computers used mainly by large organizations for critical applications, typically bulk data processing such as census, banking,  industry and consumer statistics, and financial transaction processing.

The term originally referred to the large cabinets 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 firmly established in the 1960s and most large computers were based on architecture established during that era up until the advent of Web servers in the 1990s. (The first Web server running anywhere outside Switzerland ran on an IBM mainframe at Stanford University as early as 1991. See history of world wide web for details.)

There were several minicomputer operating systems and architectures that arose in the 1970s and 1980s, but minicomputers are generally not considered mainframes. (UNIX arose as a minicomputer operating system; Unix has scaled up over the years to acquire some mainframe characteristics.)