Terminology
A mainframe is an organization's central data repository (»hub«).
It is accessed by less powerful devices: workstations and terminals.
Historical development
1954: IBM 705
1959: IBM 1401
IBM 1401 (»Model T«), probably the first mass-produced digital, all-transitorized business computer that was affordable by some companies.
1964: IBM System/360 (»S/360«) starts the third generation.
S/3690 were superseded by the System z machines.