Virtual machine

A virtual machine (VM) is a computer that exists as software. It has what looks like its own processor, memory, disk and network card, runs its own operating system and its own programs, and as far as that software can tell, it is a real machine. Underneath, it is sharing a physical computer with other virtual machines.

A layer called a hypervisor makes this possible: it splits the real CPU, RAM and disks into slices and gives each VM its own, keeping them isolated so one cannot see or crash another. That lets you run Windows inside a Mac, test software on several systems at once, or keep an old program alive on an operating system that no longer runs on modern hardware.

VMs are the foundation of cloud computing. When you launch an Amazon EC2 instance or a Google Compute Engine machine, you get a virtual machine on a server in someone's data center, ready in a minute and billed while it runs. Containers are a lighter alternative: they share the host's operating system instead of each bringing their own, so they start faster but are less isolated.