At one time a computer was simply a computer. When, in the mid-1960s, firms such as DEC and Data General began marketing their mini-computers, the term “mainframe” (also known as “big iron”) arose to designate the larger general purpose machines provided by firms such as IBM, Univac, etc.

While there is no exact definition, it usually implies a machine capable of large-scale computing tasks demanding high availability, high I/O rates, large storage for online databases, and sometimes even the ability to run multiple operating systems simultaneously.