Hypervisor
also: virtual machine monitor
The software layer that creates and runs virtual machines, sharing one physical machine’s hardware among them.
A hypervisor (virtual machine monitor) is the software layer that creates, runs, and isolates virtual machines, allocating the physical CPU, memory, and I/O among them. Type 1 hypervisors run directly on the hardware (bare-metal, e.g. ESXi, KVM) for cloud and production; Type 2 run on top of a host OS (VirtualBox) for desktops.
Worked example: a cloud host runs a Type 1 hypervisor that schedules many customer VMs onto the physical CPUs and enforces that one VM cannot read another’s memory — the mechanism that makes multi-tenant cloud safe. Gotcha: the hypervisor is the trust boundary of virtualization, so a hypervisor escape (a VM breaking out to the host or another VM) is a catastrophic class of vulnerability — which is why cloud providers patch them urgently and why some high-security workloads still demand dedicated hardware.