System z is IBM&#x27;s mainframe architecture (the hardware).<p>z&#x2F;OS is the flagship operating system. It is at its heart a batch-oriented system, but it does have a Unix subsystem (called USS for &quot;Unix System Services&quot;, formerly known as MVS OpenEdition or OMVS) that is in fact certified against a rather old version of the Single Unix Specification.<p>So in a way, z&#x2F;OS is in fact a Unix system, but that is kind of like saying that Windows is a Unix due to Cygwin or Microsoft&#x27;s Services for Unix. Mostly, it is very non-Unix-like.<p>Interestingly, IBM also offers a number of other operating systems for System z, including VM (although that system apparently does little but host virtual machines), TPF (a &quot;real-time&quot; transaction processing system) and zVSE (formerly known as DOS&#x2F;360, another, smaller batch-oriented system). And, of course, Linux is available from a number of distributions. Oh, and just for good measure, Open Solaris was ported to run inside VM, although I am not sure what became of that or if anyone is actually using it.