noun
guest OS; guest operating system
Computing term for an operating system running inside a virtual machine, as opposed to the host OS.
仮想マシン上でゲストＯＳとしてUbuntuを動かしている。
I'm running Ubuntu as a guest OS on a virtual machine.
ホストＯＳ is the operating system that runs directly on the physical hardware and hosts virtual machines, while ゲストＯＳ is the OS running inside a virtual machine.
From English 'guest OS', a computing term.