Source : Free On-Line Dictionary of Computing

OS/VS2
     
         An {operating system} from {IBM}, released
        with the {IBM S/370} and originally known as {OS/MVT}.  OS/VS2
        R1 was known as SVS (Single Virtual Storage) as it had a
        single 16 MB {virtual address space}.  OS/VS2 R2 was known as
        MVS - {Multiple Virtual Storage}.
     
        (1999-01-13)