The Free On-line Dictionary of Computing (19 January 2023):

Virtual Machine/Conversational Monitor System
Conversational Monitor System
VM/CMS

    (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 APIs 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)