Mainframe 101

When we say "mainframe" we mean the large enterprise-class computing systems from IBM, currently known as IBM zSeries, but historically known by such names as System/360, System/370, or System/390 and the plug-compatible machines from other vendors (which are no longer in the market). Other vendors have servers that they sometimes refer to as "mainframes" but which run some operating system other than z/OS, z/VM or z/TPF. Those may be fine machines, but they're not the ones we talk about.