z/OS is a 64-bit server operating system from IBM. It is the new name for the existing mainframe system OS/390, combining MVS and UNIX System Services (a mainframe implementation of Unix, formerly known as MVS Open Edition.)

(In the same December 2001 rebrand:

• S/390 became zSeries, the "z" standing for "zero down-time";
• AS/400 became iSeries, but OS/400 remained OS/400, rather than becoming "i/OS";
• RS/6000 became pSeries, the "p" standing for "performance", but AIX remained AIX, rather than becoming "p/OS";
• Netfinity became xSeries.)