<operating system> An IBM mainframe operating system, featuring integrated MVS, UNIX, LAN, distributed computing and application enablement services through its base elements. These base services enable open, distributed processing and offer a foundation for object-ready application development. The OS/390 base includes a Communication Server that includes VTAM, the VTAM AnyNet feature, TCP/IP and TIOC. It provides SNA (3270), APPC, High Performance Routing, ATM support, sockets and RPC.
OS/390 is basically rebranded, repackaged MVS/OE, CMOS processors, RAMAC disk arrays and open systems extension to networking in VTAM, the principle being that if you can't compete, rebrand what you have and tell everyone it's something new.