A computer system to which several of the following points apply can be described as "virtual reality" (VR):

The system includes a naturalistic, three-dimensional model of part of the outside world.

Interactivity; the system reacts to user input.

Real-time movement - as opposed to pre-produced animations - is possible. Users can decide freely at any time where they want to go within the model.

Presentation is by other means than just on a computer screen. Users should experience a feeling of "immersion" into the model space.