[StudyAnimal]

It refers basically only to the IBM z/Architecture machines. (I think no one else makes them any more. There were some IBM compatible (heh) Japanese ones recently). You can call a cluster of PCs a supercomputer, but you cant call it a mainframe.(But you can run a cluster, or sysplex, of mainframes. It sounds over the top but it is actually quite common.) They are particularly considered mainframes when they run z/OS and work in batch mode especially running CICS or IMS based applications. As soon as it runs linux, or web services, or more interactive things, the same zSeries machine is slightly less mainframy. (Using USS is less mainframy than using TSO or ISPF btw heh)