In the OpenShift documentation, we have been referring to "IBM System Z". We also have references to "s390x"

The following email forwarded by Nicole describes the correctly terminology.

Hi Nicole, that's an interesting question. Wikipedia [1] actually gives an accurate definition

IBM Z is a family name used by IBM for all of its z/Architecture mainframe computers from the Z900 on.

In July 2017, with another generation of products, the official family was changed to IBM Z from IBM z Systems;

the IBM Z family now includes the newest model the IBM z15, as well as the z14 and the z13 (released under the IBM z Systems/IBM System z names), the IBM zEnterprise models (in common use the zEC12 and z196), the IBM System z10 models (in common use the z10 EC), the IBM System z9 models (in common use the z9EC) and IBM eServer zSeries models (in common use refers only to the z900 and z990 generations of mainframe).

"s390x" is more for the Linux kernel architecture. Historically the Linux kernel architecture designations were "s390" and "s390x" to distinguish between the 31-bit and 64-bit Linux on Z kernels respectively, but "s390" now also refers generally to the one Linux on Z kernel architecture [2].