In optics, a virtual image is the reflection that you see when you stand in front of a
regular mirror. (See, for example, the cartoon at the beginning of Chapter 18.) You know
that the image isn’t real. For one thing, it’s behind the wall that the mirror is mounted on.
For another, you can’t touch it. In early, time-shared computing, a large central computer
commonly supplied computing services to users at terminals located remotely from the
computer. In a sense, it seemed as though the user had access to a computer that was all her
own. Starting in the early 1970s, IBM offered the VM (virtual machine) operating system
to support this concept.