I don't think there is real definition of a
Mainframe, its what you want
it to be at the time....
What is especially funny is when computer ILLiterates, such as college
administrators, think that it is two words "main frame", and think that
that means the box that the computer is in, aka "the Tower". ("That
Computer Hardware class should teach people how to open the tower, and
replace boards in the main frame, so that they can fix things like
"File Not Found". When I was in the classroom, I often had to fix
things in the main frame.") (They are why same connectors can never
be used for multiple things!)
(Our district DID have an IBM mainframe with CICS accessible with 3270
emulation)