I always thought of the distinctions this way (from my basis of exposure from late 1970s
through the 1980s) and from a higher educational setting primarily:
Mainframe = repairs required multiple technicians, some possibly there full-time; regular
operator(s) present, and a locked door located between you and the machine; entire
specialized room with raised flooring, extra-high amperage specialized power sources and
wiring, and significant air conditioning
Minicomputer = Vendor still provides a technician (just one) for repairs, who drives in
out in a station wagon; only a part-time operator only; an user can be located in the same
room; 240-volt wiring, but not particularly outlandish
Microcomputer = Computer can sit on a desk or in a "normal" room; broken
computer taken by user to someplace to be repaired or self-repaired; typically one user,
and only 120-volt household or office power needed.
Supercomputer = a really fast and specialized version (primarily focusing on high-speed
mathematical computations) of a mainframe.
Kevin Anderson