Mainframes used to just be called COMPUTERS !!
Some of us still remember a time before the PDP-8. When MINIcomputers arrived, mainframes then became known as MAINFRAMES to distinguish themselves from their smaller, departmental brethren.
When MICROcomputers arrrived, there was an 8-bit/ 16-bit / 32-bit breakdown, but nothing was called a SERVER until the early 80's and the early networking involved user PCs which became called CLIENTS in the early CLIENT-SERVER architectures. Prior to this period, TERMINALS talked to the computer, and there was no concept of a "client."
This is when APPLICATIONS began running on SERVERS (where previously they just did file and print duty.) Netware ran code as "loadable modules" but it was NT that really sealed the deal.
This well predates the ISeries and pSeries, and Zseries.