What is a mainframe? (was Re: NASA unplugs last mainframe)

The term 'mainframe' helped to categorize the physical aspect of machines from the 1950s through 1980s , but since then it has diminished as a differentiator. The mainframe architecture which emphasized dedicated processors to handle IO, front-end processors, support for large numbers of diverse peripheral devices, OSes which included batch and interactive services, also appears to help categorize machines into the 1980s. After that these combination of features started to appear in new systems or were added as extensions to so-called midrange systems (not only IBM), increasingly on lower cost systems. The distinctive mainframe 'style' of computing: the way of interacting with the operating system, OS design, communication protocols, legacy support - captures systems onwards beyond the 1980s till present day. This includes desktop (PC), small deskside cabinet implementations of mainframes like IBM XT/370, P/390, Unisys SCAMP (A-series, Unisys System 80), but also the newer (current day) technology implementations like the z-Series and the Unisys Libra. This criteria helps to answer the question: "What type of computing is a P/390 or Hercules Simulator showing? Answer: a mainframe style of computing, it is not a physical mainframe, but the manifestation shown is a mainframe.'. The marketing folks latched onto the mainframe term as a way to garner some of the halo effect of big, powerful, expensive ("THIS product is just as good as a mainframe but cheaper" etc), meet-the-needs-of-any-business mantra which sold them so well through the early decades. I think at various times marketeers tied themselves into knots trying to reconcile the categories. Unisys advertised the A1 as a "small-frame" computer in the late 1980s. Therefore mainframe only exists today as a identifiable category because examples of the legacy architecture (although significantly updated in technology and implementation) continue to exist. If the mainframe products of IBM and Unisys [*] disappeared tomorrow, the idea of mainframe (as we collectively know them) would fade too. [*] I expect there are other manufacturers, I am using IBM and Unisys as just two examples.