DEC's fancy name for VAX/VMS, a cool but proprietary operating system.
Homepage: http://vmssoftware.com/products.html Community: http://www.openvms.org/stories.php Wikipedia: http://en.wikipedia.org/wiki/OpenVMS
It acquired the name after DEC introduced the Alphaserver line, based on their Alpha RISC CPU, and ported VMS to it. Older VAXen based on the LSI-11 chipset just ran VMS. VMS was also ported to the Intel Itanium architecture.
VMS's influence is widespread: Dave Cutler, the former chief architect of VMS, was the principal architect of Microsoft's NT kernel, and various things pioneered by VMS are in NT.
More recently, former DEC developers involved in OpenVMS have formed VMS Software and taken over development from HP, who bought DEC after it foundered. VMS Software intends to port OpenVMS to the Intel X86 architecture.