Here's a CW article from 1975 that even headlines
the term:
http://tinyurl.com/bx5yypw
It makes interesting reading and the first paragraph is particularly germane
to the discussion.
But to be perfectly fair, it *was* the 70s.
Ngram shows both minicomputer and mainframe *far* exceed midicomputer
in usage during the 1970s. Midicomputer almost is not there.
--
Will