I'm curious about what various people think defines a mainframe. IBM people clearly have something in mind, and DEC people something else. Maybe DG had yet another definition.
A mainframe is a high-RAS (reliability, availability, serviceability), high-performance, high-cost, modular multiprocessor machine with specialized coprocessors; running a hypervisor-style operating system that is largely concerned with allocating resources and managing security partitions. Interconnections among modules happen at system bus latencies and bandwidths.