A piece of software that makes a single computer act as if it is many computers; that is, act as if many “virtual machines” are available rather than just the single “physical” machine. It is a core component of datacenters today and of the cloud; an AWS datacenter has many many “racks” of servers, and each of those servers has a hypervisor that—in effect—multiplies the number of servers available to customers.
— War and Peace and IT: Glossary, Mark Schwartz