DEC's fancy name for VAX/VMS, a cool but proprietary operating system.
Homepage: http://h71000.www7.hp.com/ Community: http://www.openvms.org/stories.php
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'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.