Xen (https://www.xenproject.org/) is a virtualization project that allows a single physical machine to run multiple virtual servers. Each virtual server thinks it owns an entire computer (kinda, there are ways around it).
The machine which holds the virtuals is commonly referred to as the DOM0, and the virtuals are commonly referred to as DOMU's
I have included Xen under the linux menu since it was originally strongly oriented towards Debian Linux, though it can also be installed on various other unicies.