User-mode Linux

User-mode Linux (UML) is a virtualization system for the Linux operating system based on an architectural port of the Linux kernel to its own system call 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 um architecture can then boot as a process under another Linux kernel, entirely in user space, without affecting the host environment's configuration or stability.

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.