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Definition: z/OS

IBM's flagship operating system for its mainframes. Introduced in 2000, z/OS is Unix compliant. It runs traditional IBM mainframe software, such as CICS, IMS and SNA, along with Unix/Linux mainstays such as Java and TCP/IP. Earlier versions of z/OS ran in System/390 mainframes with 31-bit addressing, but after z/OS Version 1.5 in 2007, z/OS only ran on 64-bit System z machines. See IBM Z.