Often abbreviated as ZOS, z/OS is IBM's mainframe operating system. It is a 64-bit operating system that supports Java, UNIX, API's, and TCP/IP to name a few features. z/OS.e is a specially priced, slimmed-down offering of z/OS. It provides select functions but uses the same code base as z/OS and invokes an operating environment that is comparable to z/OS in service, management, reporting, and reliability. The first name for IBM's mainframe operating system was MVS (Multiple Virtual Storage), which was succeeded first by OS/390 and then z/OS.
See also MVS.