Source : Free On-Line Dictionary of Computing

Virtual Machine/Conversational Monitor System
     
         (VM/CMS) An {IBM} {time-sharing} and
        personal computing environment executing under {Virtual
        Machine} (VM) in a virtual machine environment.  VM/CMS is
        designed to support large numbers of {interactive} users.  It
        relies on numerous {API}s into the {Control Program} (CP) to
        provide very efficient single-user processing
     
        VM/CMS was only adopted some time after the original design of
        {Virtual Machine} as a more efficient personal computing
        environment than {MVS/TSO}.
     
        (1999-01-19)