A virtual machine monitor (VMM) is a host program that allows a single computer to support multiple, identical execution environments. All the users see their systems as self-contained computers isolated from other users, even though every user is served by the same machine. In this context, a virtual machine is an operating system (OS) that is managed by an underlying control program. For example, IBM's VM/ESA can control multiple virtual machines on an IBM S/390 system.
In Microsoft Virtual Server 2005, Virtual Machine Monitor is the proprietary name for a kernel-mode driver that functions as a firewall between the host OS and the virtual machines. It can prevent any single program, running in one of the virtual machines, from overusing the resources of the host OS.