Philippe Fuchs' Technical definition or VR

You may talk about VR for a system if and only if ...

The man/user is an observer and actor (meaning he has an action on ...)

Something to explore, fictitious or not

User action, Navigation; RT : the user perception

The user should  not perceive the delay between its action (motor) and the sensory response from the system. The max delay is about 100 ms (depends on which direction)(25 fps <=> 40ms)

Feeling of being somewhere

The "natural" is what you have learnt in the real life ... Even if we once achieve to reach the same level of complexity (and some think it is not possible), one may wonder if we really need to reach the perfection to achieve efficiency in regards to the expected result.

Input and Output, sensory-motor interface, Metaphors

To make a difference from the set "screen, keyboard and mouse" which match with the Human Computer Interface field ...

A Behavioural Interface is an apparatus that involve a human behaviour, natural and without (or with short) learning period.

All the senses may be taken into account, not all are needed for all the applications

NB : No "Why" (Answer : the Applications)

Other definitions often mix purpose, what it is and what it is for (applications). Moreover, one should not define VR by the tools it uses (Head mounted display, dataglove ...)

Not these all 4 points will be perfectly done in all VR application, but the 4 should just appear if we want to be able to talk about a VR project.