Run many computers on one computer: that is the promise of virtualization. A hypervisor such as KVM, Xen, Hyper-V, or VMware ESXi divides a host's processors, memory, and storage into guests, each with its own operating system, walled off from the rest. Boot one in seconds, resize it, snapshot it, move it to another machine.

The idea underpins modern infrastructure. Public clouds rent virtual machines carved from shared servers, enterprises fold racks of hardware into a handful of hosts, and developers test across operating systems on a laptop. Abstract the hardware, isolate the workload, put every core to use: the word describes the mechanism itself.