I'll take a stab at a current definition. A mainframe is a high-RAS (reliability, availability, serviceability), high-performance, high-cost, modular multiproc

I'll take a stab at a current 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.