Mainframes (often colloquially referred to as Big Iron) are computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry and consumer statistics, ERP, and financial transaction processing.
The term probably had originated from the early mainframes, as they were housed in enormous, room-sized metal boxes or frames. Later the term was used to distinguish high-end commercial machines from less powerful units.
Today in practice, the term usually refers to computers compatible with the IBM System/360 line, first introduced in 1965. (IBM System z10 is the latest incarnation.) Otherwise, large systems that are not based on the System/360 are referred to as either "servers" or "supercomputers". However, "server", "supercomputer" and "mainframe" are not synonymous (see client-server).