(MVS) Release 2 of OS/VS2, called MVS because it had multiple 16 MB virtual address spaces, in contrast to SVS. MVS ran on the IBM 390 series mainframes. It became MVS/SP, then MVS/XA (with 31-bit addressing) and then MVS/ESA.
MVS/Open Edition (MVS/OE), aimed at the growing open systems market, added TCP/IP and Unix support in an MVS address space, allowing users to run IBM, CICS-type applications, batch applications and Unix.
MVS/ESA was repackaged as OS/390 as a marketing exercise but it's basically the same thing.