Mainframe computers were developed in the early 1950's when computers were made that size. It means the cabinet more than the contents as early ones had the CPU (Central Processing Unit) in a separate cabinet.

A personal computer

is a computer meant for home users and is typically a single CPU with a maximum of a quad core technology whereas a mainframe is a computer which is meant for organizations and is typically comprised of thousands of processors.

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short history of mainframe

The IBM zEnterprise 196 mainframe