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User-mode Linux
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<p>User-mode Linux (UML) is a <a href="/facts/Virtualization/bEqH0Wtf">virtualization</a> system for the <a href="/facts/Linux/9FvCKSTq">Linux</a> operating system based on an architectural <a href="/facts/Porting/7s6tFs8B">port</a> of the <a href="/facts/Linux_kernel/dlkduHuA">Linux kernel</a> to its own <a href="/facts/System_call/NW44AFJr">system call</a> interface, which enables multiple virtual Linux kernel-based operating systems (known as guests) to run as an application within a normal Linux system (known as the host). A Linux kernel compiled for the <i>um</i> architecture can then boot as a process under another Linux kernel, entirely in <a href="/facts/User_space_and_kernel_space/0yIzMY8t">user space</a>, without affecting the host environment's configuration or stability. </p><p>This method gives the user a way to run many virtual Linux machines on a single piece of hardware, allowing some isolation, typically without changing the configuration or stability of the host environment because each guest is just a regular application running as a process in user space. </p>