An OperatingSystem (often Linux or Solaris) set up in a VirtualMachine or something akin to it, allowing to run a multitude of VirtualPrivateServers on a single computer. This is a very useful technique for ISPs and other 3rd party server providers, which can consolidate many servers onto BigIron, making their hardware as well as their customers' servers easier to manage.
RimuHosting offers Linux VPSs.
Various software projects exist that implement this strategy:
Linux-VServer